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. "Crystallographic Dependence of Visible-Light Photochemistry in Epitaxial TiO2-xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Art. No. 085401. Abstract All films were grown by plasma assisted molecular beam epitaxy (PAMBE) in a custom chamber described elsewhere (1). Epitaxial films of TiO2-xNx(001) (x ≤ ~0.02) anatase were grown by PAMBE on undoped or Nb-doped (0.02 at. %) SrTiO3(001) (STO) and undoped LaAlO3(001) (LAO). Similarly, TiO2-xNx(001) (x ≤ ~0.02) rutile epifilms were grown on rutile TiO2(110). The growth and physical properties of N-doped anatase on LAO(001) and N-doped rutile on TiO2(110) have been described in detail elsewhere (2-4). In what follows, we describe the growth details for N-doped anatase on STO(001). The PAMBE chamber is connected to an x-ray photoelectron spectrometer (XPS) chamber and a photodesorption chamber. The former is equipped with a Gamma Data/Scienta SES 200 analyzer and a monochromatic AlK x-ray source. The latter includes a molecular dosing apparatus for TMAA, a Hg arc lamp, and a quadrupole mass spectrometer. The STO substrates were etched in buffered HF and annealed in flowing O2 at 1 atm. at 950oC for 8 hours. The etch dissolved SrO terraces and the oxygen anneal resulted in mass transport of the discontinuous TiO2 microterraces, resulting in an atomically flat, TiO2 terminated surface with a minimum step height of 4 Å (5). This treatment left some residual fluorine on the surface which could not be removed by annealing. The measured F 1s binding energy was ~684.0 eV, which is close to that exhibited by SrF2 – 684.6 eV (6). Based on this binding energy and the high degree of thermal stability, we conclude that F substitutes for O in the lattice. Under this assumption and using atomic photoemission cross sections (7), the F mole fraction within the anion sublattice is estimated to be ~0.05 within the probe depth of XPS at normal emission (~45 Å).
2009. "Crystallographic Dependence of Visible-light Photoactivity in Epitaxial TiO2−xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Article number: 085401. doi:10.1103/PhysRevB.79.085401 Abstract Nitrogen-doped TiO2 materials have been shown to exhibit visible-light photoactivity, but the operative mechanism(s) are not well understood. Here we use structurally and compositionally well-defined epitaxial films of TiO2−xNx anatase (001) and rutile (110) (x~0.02) to show a qualitative difference between the visible-light activities for the two polymorphs. Holes generated by visible light at N sites in anatase (001) readily diffuse to the surface and oxidize adsorbed trimethyl acetate while the same in rutile (110) remain trapped in the bulk. In light of the low doping densities that can be achieved in phase-pure material, conventional wisdom suggests that holes should be trapped at N sites in both polymorphs. Although the detailed mechanism is not yet understood, these results suggest that the hole hopping probability is much higher along the [001] direction in N-doped anatase than along the [110] direction in N-doped rutile.
2009. "Growth and characterization of barium oxide nanoclusters on YSZ(111)." Journal of Physical Chemistry C 113(32):14324-14328. doi:10.1021/jp9020068 Abstract Barium oxide (BaO) was grown on YSZ(111) substrate by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). In-situ reflection high-energy electron diffraction, ex-situ x-ray diffraction, atomic force microscopy and x-ray photoelectron spectroscopy have confirmed that the BaO grows as clusters on YSZ(111). During and following the growth under UHV conditions, BaO remains in single phase. When exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O. However, in a few attempts of BaO growth, XRD results show a fairly single phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions results in the formation BaCO3 on the surface and partly Ba(OH)2 throughout in the bulk. Based on the observations, it is concluded that the BaO nanoclusters grown on YSZ(111) are highly reactive in ambient conditions. The variation in the reactivity of BaO between different attempts of the growth is attributed to the cluster size.
2009. "Morphology, Orientation Relationship and Stability Analysis of Cu2O nanoclusters on SrTiO3 (100) ." Applied Physics Letters 95(5):Art. No. 053111. doi:10.1063/1.3193530 Abstract Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and theoretical studies based on classical nucleation theory have been used to understand the morphology, orientation relationship and stability of Cu2O nanoclusters on SrTiO3 (100) (STO). We propose that the competing interfacial and elastic energies facilitate an in-plane rotation of the Cu2O clusters by 45o with respect to the STO substrate and stabilize Cu2O clusters on STO(100) with an orientation relationship of (001) Cu 2o //(001) SrTiO3 and <100> Cu 2o //<110> SrTiO3. Preliminary theoretical analysis also suggests that this particular orientation results in smaller critical nucleus sizes and lower nucleation barriers. The study also indicates a chemical potential (growth rate) dependence of the orientation relationship.
2009. "Growth-Rate Induced Epitaxial Orientation of CeO2 on Al2O3(0001)." Applied Physics Letters 94(20):204101:1-3. doi:10.1063/1.3139073 Abstract High-quality ceria (CeO2) films were grown on sapphire (Al2O3) (0001) substrates using oxygen plasma-assisted molecular beam epitaxy. The epitaxial orientation of the ceria films has been found to be (100) and (111) at low (< 8 Å/min) and higher growth rates (up to ~30 Å/min), respectively. Evidence shows that CeO2 (100) film grows as three-dimensional islands, while CeO2 (111) proceeds with layered growth. Three in-plane domains at 30° to each other are observed in the CeO2 (100), which is attributed to the close match of the oxygen sub-lattices in the film and substrate that has a three-fold symmetry. Molecular dynamic simulations have further confirmed that the CeO2 film retains (100) orientation on the Al2O3 (0001) substrate.
2009. "Synthesis and Characterization of Bulk Vitreous Cadmium Germanium Arsenide." Journal of the American Ceramic Society 92(6):1236-1243. doi:10.1111/j.1551-2916.2009.03001.x Abstract Abstract Cadmium-germanium-diarsenide (“CGA”) glasses were synthesized in bulk form (~2.4 cm3) using the procedures adapted from the literature. Several issues involved in the fabrication and quenching of amorphous CdGexAs2 (x = 0.45, 0.65, 0.85, 1.00) are described. An innovative processing route is presented to enable quenching of vitreous, crack-free ingots with sizes up to 10 mm diameter, and 30 – 40 mm long. Specimens from selected ingots were characterized using thermal analysis, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, particle-induced X-ray emission, Rutherford backscattering, secondary ion mass spectrometry, X-ray diffraction, density, and optical spectroscopy. Variations in properties as a function of processing conditions and composition are described. Results show that the density of defect states in the middle of the band gap and near the band edges can be decreased three ways: through suitable control of the processing conditions, by doping the material with hydrogen, and by increasing the concentration of Ge in the glass.
2009. "Suppression of conductivity in Mn-Doped ZnO Thin Films." Journal of Applied Physics 105(1):013715. doi:10.1063/1.3063730 Abstract We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition (MOCVD). The ion beam, surface and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was noticed up to ~ 4.5 atom% Mn doping. The presence of Mn2+, confirmed by X-ray photoelectron spectroscopy (XPS), is correlated with the reduction in conductivity. No major change in the activation energy (~40 meV) and a reduction in the Zn/O ratio as a function of Mn concentration in highly sensitive proton induced X-ray emission (PIXE) technique also support this hypothesis. We discuss our results from a view point of homogeneous Mn distribution, elemental XPS ratio offsets and secondary phase formations in ZnO films.
2009. "X-ray absorption fine structure and magnetization characterization of the metallic Co component in Co-doped ZnO thin films ." Physical Review. B, Condensed Matter 79(7):Art. No. 075202. Abstract X-ray absorption fine structure (XAFS) measurements have been used to characterize a series of Co doped ZnO films grown on sapphire substrates by pulsed laser deposition. The emphasis is on characterization of the fate of the Co dopant: metallic particles or substitutional Co2+. It is shown that analysis of both the near edge and extended fine structure can provide a measurement of the fraction of metallic Co. Any quantitative understanding of magnetism in this system needs to take account of both types of Co. Results are reported for two types of films from two different groups that show distinctly different behavior. Films grown with high concentrations of Co show varying amounts of metallic Co that could be identified as hcp or fcc Co. Another set of films were annealed in Zn vapor to induce magnetism. These also showed significant metallic Co, but of a different type similar to the CoZn intermetallic. The bulk forms of both metals are magnetic and should contribute to the magnetism. Using bulk magnetic values, there are some discrepancies with room temperature magnetic measurements. The 2 magnetic properties of the small metal particles are likely changed by their surroundings and by superparamagnetism. Low temperature magnetic measurements for one of the samples confirmed this with an estimated blocking temperature of 50K.
2009. "Influence of samaria doping on the resistance of ceria thin films and its implications to the planar oxygen sensing devices." Sensors and Actuators. B, Chemical 139(2):380-386. doi:10.1016/j.snb.2009.03.021 Abstract In order to evaluate and analyze the effect of samarium (Sm) doping on the resistance of cerium oxide, we have grown highly oriented samaria doped ceria (SDC) thin films on sapphire, Al2O3 (0001) substrates by using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The film growth was monitored using reflection high-energy electron diffraction (RHEED) which shows two-dimensional growth throughout the deposition. Following growth, the thin films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution X-ray diffraction (HRXRD), and Rutherford backscattering spectrometry (RBS). XPS depth-profile shows Sm atoms are uniformly distributed in ceria lattice throughout the bulk of the film. The valence states of Ce and Sm in doped thin films are found to be Ce4+ and Sm3+, respectively. HRXRD shows the samaria doped ceria films on Al2O3(0001) exhibit (111) preferred orientation. Ion-channeling in RBS measurements confirms high quality of the thin films. The resistance of the samaria doped ceria films, obtained by two probe measurement capability under various oxygen pressure (1mTorr-100Torr) and temperatures (623K to 973K), is significantly lower than that of pure ceria under same conditions. The 6Sm% doped ceria film is the optimum composition for highest conductivity. This is attributed to the increased oxygen vacant sites in fluorite crystal structure of the epitaxial thin films which facilitate faster oxygen diffusion through hopping process.
2009. "On the relationship between non-stoichiometry and passivity breakdown in ultra-thin oxides: combined depth-dependent spectroscopy, Mott-Schottky analysis and molecular dynamics simulation studies." Journal of Physical Chemistry C 113(9):3502-3511. Abstract Understanding the relationship between non-stoichiometry and physical properties of ultra-thin oxides is of great importance from both scientific and technological aspects. A specific example includes the onset of passivity breakdown in an ultra-thin oxide film in aqueous medium leading to the onset of corrosion. In this work, using the model system of ultra-thin oxide of alumina on aluminum synthesized by natural oxidation and photon-assisted oxidation processes; we demonstrate a direct correlation between passivity and quality of the oxide film quantitatively. Depth-dependent high resolution X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and nuclear reaction analysis (NRA) have been performed to characterize the physical and chemical properties of the oxide films, while detailed impedance measurements and Mott-Schottky studies have been performed to understand electronic transport. Combined NRA and TEM analysis reveal an 18% increase in oxygen density (for oxide films with near identical thicknesses ~3.8nm) in case of photon-assisted oxidation. The denser oxide film results in a ~34% more blockage of chloride ions transport as indicated by XPS analysis. Mott Schottky measurements on these oxide films indicates a 43% reduction of defect levels for UV-synthesized alumina when compared to native one, suggestive of chloride ion transport via oxygen vacancies. Additionally, molecular dynamics simulations have been performed to provide insights into the structure of the oxides at the atomic level to correlate with the experimental measurements. These simulations employ dynamic charge transfer between atoms and are used to investigate nanoscale oxides grown on Al (100) surfaces due to atomic and molecular oxygen. Oxidation using molecular and atomic oxygen resulted in an amorphous oxide scale with self limiting thickness of ~ 16 and 22 Å, respectively at 300 K. Structural and dynamic correlations indicate significant charge transfer to exist in the oxide film in both the cases. The oxide growth in both the cases occurs due to the inward oxygen and outward cation diffusion. The calculated in-plane and out-of-plane atomic diffusivities are 40-70% higher in case of atomic oxidation. In the presence of atomic oxygen, the O/Al ratio is more uniform and varies from 1.37 at the oxide-gas interface to 1.30 at metal-oxide interface whereas that formed by natural oxidation was sub-stoichiometric and oxygen deficient with O/Al values varying from 1.27 (oxide-gas interface) to 1.05 (metal-oxide interface) at room temperature. The simulation results are consistent with the reported experimental investigations.
2009. "Mexico City Aerosol Analysis during MILAGRO using High Resolution Aerosol Mass Spectrometry at the Urban Supersite (T0). Part 1: Fine Particle Composition and Organic Source Apportionment." Atmospheric Chemistry and Physics 9(17):6633-6653. Abstract Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and complementary instrumentation. Mass concentrations, diurnal cycles, and size distributions of inorganic and organic species are similar to results from the CENICA supersite in April 2003 with organic aerosol (OA) comprising about half of the fine PM mass. Positive Matrix Factorization (PMF) analysis of the high resolution OA spectra identifies three major components: chemically-reduced urban primary emissions (hydrocarbon-like OA, HOA), oxygenated OA (OOA, mostly secondary OA or SOA), and biomass burning OA (BBOA) that correlates with levoglucosan and acetonitrile. BBOA includes several very large plumes from regional fires and likely also some refuse burning.
2008. "Growth and structure of epitaxial Ce0.8Sm0.2O1.9 by oxygen-plasma-assisted molecular beam epitaxy." Journal of Crystal Growth 310(2008):2450-2456. Abstract The epitaxial growth of Ce0.8Sm0.2O1.9 films on sapphire (0001) substrate by oxygen-plasma-assisted MBE has been characterized using RHEED, XPS, XRD, AFM, HRTEM and RBS in order to determine their structure and compositions. The composition of the films was determined to be Ce: Sm: O of 0.8:0.2:1.9 by RBS. The film/substrate epitaxial relationship can be written as CeO2 (111)// -Al2O3 (0001) and CeO2 [110]// -Al2O3 . The Ce has only 4+ oxidation state in the films and Sm is fully oxidized in the films with formal oxidation of 3+. CeO2 (111) face is preferred orientation and the thin films are cubic phases.
2008. "Conductivity of Oriented Samaria-Doped Ceria Thin Films Grown by Oxygen-plasma-assisted Molecular Beam Epitaxy." Electrochemical and Solid-State Letters 11(5):B76-B78. doi:10.1149/1.2890122 Abstract We have used oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) to grow highly oriented Ce1-xSmxO2-δ films on single crystal c-Al2O3. The samarium concentration, x, was varied in the range 1-33 atom%. It was observed that dominant (111) orientation in Ce1-xSmxO2-δ films can be maintained up to about 10 samarium atom% concentration. Films higher than 10 atom% Sm concentration started to show polycrystalline features. The highest conductivity of 0.04 S.cm-1, at 600 0C, was observed for films with ~ 5 atom% Sm concentration. A loss of orientation, triggering an enhanced grain boundary scattering, appears to be responsible for the decrease in conductivity at higher dopant concentrations.
2008. "Photoluminescence of SnO2 nanoparticles embedded in Al2O3." Journal of Physics D. Applied Physics 41(22):225102. doi:10.1088/0022-3727/41/22/225102 Abstract Tetragonal Sn nanoparticles of ∼15 nm diameter are produced in Al2O3 by direct Sn implantation at room temperature. After thermal annealing at 1000 ◦C in oxygen, the implantation-induced amorphous region recrystallized and the Sn nanoparticles turned into SnO2 nanoparticles with an average diameter of ∼30 nm as revealed by transmission electron microscopy. While no absorption and photoluminescence (PL) are observed from the metallic Sn nanoparticles, SnO2 nanoparticles exhibit an absorption edge at ∼280 nm and three emission bands at 410 nm, 520 nm and 700 nm, respectively. In addition to the previously reported blue and green emission from SnO2 nanostructures, a red PL band was observed due to the unique surface state of SnO2 nanoparticles embedded in Al2O3 substrate fabricated by ion implantation.
2008. "Direct Measurement of Oxygen Incorporation into Thin Film Oxides at Room Temperature Upon Ultraviolet Phton Irradiation." Applied Physics Letters 93(26):263109. doi:10.1063/1.3058691 Abstract Minute changes in oxygen concentration in complex oxides even of the order of ~0.001% can significantly influence functional properties ranging from onset of superconductivity to colossal dielectric constant and ferroic response. In this letter, we report on direct experimental measurement of enhanced oxygen incorporation into ultra-thin oxide films at room temperature under gentle UV photon exposure. Oxygen concentration changes in nanoscale yttria-doped-zirconia (YDZ) films grown on Ge substrate were quantified using the 16O(d,p)17O nuclear reaction. The oxygen concentration was consistently ~ 3 % larger in UV irradiated YDZ films compared to as-grown YDZ films and can be kinetically controlled. Possible incorporation mechanisms are discussed. This suggests a novel approach to modulate oxygen concentration in complex oxides. There is tremendous interest in the science and applications of ultra-thin oxide films, such as electrolyte membranes for solid oxide fuel cells 1, high-dielectric constant (high-) oxides for metal-oxide-semiconductor (MOS) devices 2 and multi-ferroics 3. In addition, thin film oxides also serve as model systems to investigate space charge effects on carrier transport and strongly correlated phenomena such as phase transitions. An overarching problem of central importance is the controlled synthesis of oxide films and how they impact functional properties. Particularly, the role of oxygen vacancies or non-stoichiometry has been found to be crucial in this regard. Examples include large magneto resistance effect and metal-to-insulator transition introduced by reducing oxygen stoichiometry of poly crystalline La0.67Ba0.33MnOz 3, nonsuperconducting-to-superconducting transformation by minute amount of oxygen incorporation upon annealing YBa2Cu3O7-(YBCO)films 4, and blue light emission at room temperature in oxygen deficient SrTiO3 (STO) 5, 6. These studies revealed broad spectra of properties with subtle changes (e.g., of the order of ~0.01% to introduce semiconductor-to-metal transition in SrTiO3)7 in oxygen-related defects, thereby it is extremely important to develop an understanding of oxygen concentration in thin films.
2008. "Oxidation and Metal-Insertion in Molybdenite Surfaces: Evaluation of Charge-Transfer Mechanisms and Dynamics ." Geochemical Transactions 9(8):, doi:10.1186/1467-4866-9-8 Abstract Molybdenum sulfide (MoS2), an important representative member of the layered transition-metal dichalcogenides, has been of special importance to the research community of geochemistry, materials and environmental chemistry, and industrial science and technology. Understanding the oxidation behavior and charge-transfer mechanisms in MoS2 is important to gain better insight into the degradation of this mineral in the environment. On the other hand understanding the insertion of metals into molybdenite and evaluation of charge-transfer mechanism and dynamics is quite important to utilize these minerals in technological applications. Furthermore, such a detailed investigation of thermal oxidation behavior and intercalation process will provide a basis to further explore and model the mechanism of adsorption of metal ions on to geomedia. Therefore, the present work was performed to understand the oxidation and intercalation processes of molybdenite surfaces. The results obtained, using a wide variety of analytical techniques, are presented and discussed in this paper.
2008. "Characterization of Aerosols Containing Zn, Pb, and Cl from an Industrial Region of Mexico City." Environmental Science & Technology 42(19):7091-7097. doi:10.1021/es7030483 Abstract During the March, 2006 MILAGRO campaign, measurements in the Northern Mexico City Metropolitan Area revealed the frequent appearance of particles with a characteristically high content of internally mixed Zn, Pb, Cl, and P. A comprehensive study of the chemical and physical properties of these particles was performed using a complementary combination of aerosol measurement techniques. Individual particles were analyzed using Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) and Computer Controlled Scanning Electron Microscopy/Energy Dispersive X-Ray spectroscopy (CCSEM/EDX). Proton Induced X-Ray Emission (PIXE) analysis of bulk aerosol samples provided time-resolved mass concentrations of individual elements. The PIXE measurements indicated that Zn is more strongly correlated with Cl than with any other element and that Zn concentrations are higher than other non-ferrous transition metals. The Zn- and Pb - containing particles have both spherical and non-spherical morphologies. Many metal rich particles had needle-like structures and were found to be composed of ZnO and/or Zn(NO3)2·6H2O as indicated by scanning transmission x-ray microscopy/near edge X-ray absorption spectroscopy (STXM/NEXAFS). The Zn and Pb rich particles were primarily in the submicron size range and internally mixed with elemental carbon. The unique chemical associations most closely match signatures acquired for garbage incineration. This unique combination of complementary analytical techniques has allowed for a comprehensive evaluation of Zn- and Pb- containing particles in a complex urban environment, highlighting unique characteristics that give powerful insight into their origin.
2008. "Electronic properties of H and D doped ZnO epitaxial films." Applied Physics Letters 92(15):Art. No. 152105. Abstract ZnO epitaxial films grown by pulsed laser deposition in an ambient of H2 or D2 exhibit qualitatively different electronic properties compared to films grown in vacuum or O2, or bulk single crystals annealed in H2. These include temperature-independent resistivities of ~0.1 -cm, carrier (electron) concentrations in the 1018 cm-3 range, mobilities of 20-40 cm2/V-sec, and negligible (a few meV) activation energies for conduction. These transport properties are consistent with H (D) forming an ultra-shallow donor or conduction band states not achievable by post-growth annealing in H2.
2008. "A Study of H and D doped ZnO epitaxial films grown by pulsed laser deposition." Journal of Applied Physics 104(5):Article no. 053711. doi:10.1063/1.2975219 Abstract We examine the crystal structure, electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. We compared with pure ZnO films grown in O2 and vacuum. N-type electrical conductivity is enhanced by two to three orders of magnitude as a result of growing in H2 or D2. Temperature dependent Hall effect measurements reveal small (a few meV) carrier activation energies, along with carrier concentrations of 2-7 x 1018 cm-3, and mobilities of 20-40 cm2/Vs in ZnO films doped with H or D in the 1018 cm-3 range. We have modeled the low-temperature electrical properties of H- and D-doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 or D2 promotes the formation of an exceedingly shallow or conduction-band degenerate donor state, possibly associated with H or D substitution at O sites in the lattice.
2008. "Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films." New Journal of Physics 10:Art. No. 055010. doi:10.1088/1367-2630/10/5/055010 Abstract Epitaxial thin films of cobalt-doped ZnO (Co:ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10-4 – 105 -cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co:ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co:ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.
2008. "Comparative Analysis of Urban Atmospheric Aerosol by Particle-Induced X-ray Emission (PIXE), Proton Elastic Scattering Analysis (PESA), and Aerosol Mass Spectrometry (AMS)." Environmental Science & Technology 42(17):6619-6624. doi:10.1021/es800393e Abstract A multifaceted approach to atmospheric aerosol analysis is often desirable in field studies where an understanding of technical comparability among different measurement techniques is essential. Herein we report quantitative intercomparisons of Particle-Induced X-ray Emission (PIXE) and Proton Elastic Scattering Analysis (PESA), performed off-line under vacuum, with analysis by Aerosol Mass Spectrometry (AMS) carried out in real-time during the MCMA-2003 Field Campaign in the Mexico City Metropolitan Area. Good agreement was observed for mass concentrations of PIXE-measured sulfur (assuming it was dominated by SO42−) and AMS-measured sulfate during the most of the campaign. PESA-measured hydrogen mass was separated into sulfate H and organic H mass fractions assuming the only major contributions were (NH4)2SO4 and organic compounds. Comparison of the organic H mass with AMS organic aerosol measurements indicates that about 75% of the mass of these species evaporated under vacuum. However ~25% of the organics does remain under vacuum, which is only possible with low vapor pressure compounds, and which supports the presence of high molecular weight and/or highly oxidized organics consistent with atmospheric aging. Approximately 10% of the chloride detected by AMS was measured by PIXE, possibly in the form of metal-chloride complexes, while the majority of Cl was likely present as more volatile species including NH4Cl. This is the first comparison of PIXE/PESA and AMS, and to our knowledge also the first report of PESA hydrogen measurements for urban organic aerosols.
2008. "Radiation Effects on the Microstructure of a 9Cr-ODS Alloy." JOM. The Journal of the Minerals, Metals and Materials Society 60(1):24-28. doi:10.1007/s11837-008-0003-5 Abstract Oxide dispersion strengthened (ODS) steels are a prime candidate for high temperature, high dose cladding in advanced nuclear reactors. A 9Cr-ODS alloy was irradiated with 5 MeV Ni ions at temperatures of 500, 600 and 700ºC to doses up to 150 dpa. There was no significant change in the dislocation arrangement. For oxide particles, there is a small shrinkage in size and increase in density with increasing irradiation dose. The atom probe analysis on the unirradiated and the Ni ion irradiated 9Cr-ODS at 700ºC to 150 dpa shows similar spatial configuration of oxide particles. Radiation-induced void swelling was not identified following Ni ion irradiation. Irradiation with 1.0 MeV Kr ions at temperatures of 500, 600, 700 and 800ºC to doses of 30-50 dpa reveals the presence of voids in all the irradiation conditions and it is believed to be due to Kr ions trapped in the materials. This work confirms that oxide particles and the microstructure of the 9Cr- ODS show minimal and acceptable changes under irradiation at temperature up to 700ºC and doses up to 150 dpa.
2008. "Thermal stability and oxidation resistance of TiCrAlYO coatings on SS430 for solid oxide fuel cell interconnect applications." Surface & Coatings Technology 202(19):4820-4824. doi:10.1016/j.surfcoat.2008.04.059 Abstract Chromia-forming ferritic stainless steels are being considered for interconnects applications in planar solid oxide fuel cell (SOFC) stacks because of their low cost and physical properties. At high temperatures, ferritic steels lack environmental stability in the SOFC operating environment, and gradually degrade the cell performance. In this study, an effective, dense and well adherent TiCrAlYO coating was deposited on an SS430 alloy using the filtered arc deposition technique. High-energy ion backscattering was used to characterize the composition and the thermal stability of the coatings. The chromium volatility of the coated steel plates at 800 oC was also measured using ion beam analysis. Significant reductions in oxidation rates as well as reduced Cr volatility were observed for the coated alloys.
2008. "Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates." Thin Solid Films 516(18):6088-6094. doi:10.1016/j.tsf.2007.11.007 Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and ZrO2 (111)-buffered Al2O3 (0001) substrates using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on pure Al2O3(0001) substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3(0001) substrate and the films. However, the films, grown on a thin layer of ZrO2(111) buffered Al2O3 (0001) substrate, appears to be highly oriented. These films were characterized using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS) depth profiling. Oxygen ionic conductivity in gadolinia-doped ceria films was measured as a function of Gd concentration and these results were compared with the ion conductance data of the polycrystalline and single crystalline yttria-stabilized zirconia (YSZ).
2007. "A Mechanism For Carbon Nanosheet Formation." Carbon 45(11):2229-2234. doi:10.1016/j.carbon.2007.06.017 Abstract The growth, structure and properties of a two-dimensional carbon nanostructure-carbon nanosheet produced by radio frequency plasma enhanced chemical vapor deposition have been investigated. The effects of deposition parameters on the structure and properties of carbon nanosheets were also investigated. A growth model has been described proposing that atomically thin graphene sheets result from a balance between deposition through surface diffusion and etching by atomic hydrogen, and that the observed vertical orientation of these sheets results from the interaction of the plasma electric field with their anisotropic polarizability.
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. "Fluorine doping in dilute magnetic semiconductor Sn1–xFexO2." Journal of Materials Science 18(11):1151-1155. doi:10.1007/s10854-007-9145-4 Abstract Recent studies have reported room-temperature ferromagnetism (FM) in Fe doped SnO2. The FM in semiconductors due to transition metal doping has been argued to be carrier mediated. Fluorine (F) doping in sure SnO2 has been reported to significantly increase the carrier concentration. In this work, we investigated the role of F doping in the range from 0% to 0.79% on the FM of chemically synthesized single phase Sn1xFexO2 using X-ray diffraction, UV-Vis spectrophotometry, particle-induced X-ray emission, particle-induced gamma ray emission and magnetometry. The saturation magnetization Ms (0.03 emu/g) increased by a factor of 2.5 and the lattice volume and band gap energy decreased by 0.35 A3 and 0.2eV, respectively, with 0.67% F doping (F/Sn atom %) compared to the sample without any fluorine.
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. "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. "On the room-temperature ferromagnetism of Zn1-xCrxO thin films deposited by reactive co-sputtering." Solar Energy Materials and Solar Cells 91(15-16):1496-1502. doi:10.1016/j.solmat.2007.03.012 Abstract We report on the preparation and detailed characterization of ferromagnetic Zn1-xCrxO thin films deposited on Si substrates using reactive co-sputtering of Cr and Zn in controlled oxygen atmospheres. X-ray diffraction data showed wurtzite ZnO peaks in the ferromagnetic films prepared with lower Cr powers, whose position and intensities are influenced by Cr doping. However, samples prepared with higher Cr powers did not show ferromagnetism but displayed evidence of Cr2O3 and ZnCr2O4 phases with no ZnO phase. The magnetization is higher (saturation magnetization Ms = 18 emu/cm3) for lower Cr concentrations and decreases for higher Cr doping. The samples were investigated extensively to understand the film composition, dopant distribution, homogeneity and potential origin of the observed ferromagnetism. Particle induced x-ray emission studies were employed to determine the chemical composition as well as the Cr/Zn ratio in the films. Film uniformity and homogeneity, investigated using Rutherford backscattering spectrometry, showed a relatively uniform ZnO layer in the as-prepared samples but, in a sample annealed at 800 oC, showed some diffusion of Si from the substrate. X-ray photoelectron spectroscopy studies indicated that Cr ions are in the oxidized state, but showed changes in the binding energy and Cr concentration when measured after removing 10 nm from the surface using Ar ion sputtering. Possible origins of the observed ferromagnetic behavior are discussed based on the comprehensive characterization results.
2007. "Effect of Mn doping on the structural, morphological, optical and magnetic properties of indium tin oxide films." Journal of Materials Science. Materials in Electronics 18(12):1197-1201. doi:10.1007/s10854-007-9277-6 Abstract We report high temperature ferromagnetism in Mn (3 - 4 at.%) doped optically transparent indium tin oxide (ITO) films prepared by a sol-gel based technique. The films showed 16 - 18 nm sized uniformly distributed particles with high phase purity. Magnetic measurements yield a coercivity Hc ~ 80 Oe and a saturation magnetization Ms ~ 0.39 B/Mn2+ ion at 300 K with a Tc > 600 K for the 3.2% Mn doped ITO film. Magnetic force microscopy showed convincing evidence of a uniformly distributed ferromagnetic phase with well defined magnetic domains spread over hundreds of independent nanoparticles.
2007. "Growth and surface characterization of sputter-deposited molybdenum oxide thin films." Applied Surface Science 253(12):5368-5374. doi:10.1016/j.apsusc.2006.12.012 Abstract Molybdenum oxide thin films were produced by magnetron sputtering using a molybdenum (Mo) target. The sputtering was performed in a reactive atmosphere of argon-oxygen gas mixture under varying conditions of substrate temperature (Ts) and oxygen partial pressure (pO2). The effect of Ts and pO2 on the growth and microstructure of molybdenum oxide films was examined in detail using reflection high-energy electron diffraction (RHEED), Rutherford backscattering spectrometry (RBS), energy dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) measurements. The analyses indicate that the effect of Ts and pO2 on the microstructure and phase of the grown molybdenum oxide thin films is remarkable. RHEED and RBS results indicate that the films grown at 445 *C under 62.3% O2 pressure were stoichiometric and polycrystalline MoO3. Films grown at lower pO2 were nonstoichiometric MoOx films with the presence of secondary phase. The microstructure of the grown Mo oxide films is discussed and conditions were optimized to produce phase pure, stoichiometric, and highly textured polycrystalline MoO3 films.
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. "Synthesis and characterization of lithium-doped tin dioxide nanocrystalline powders." Materials Chemistry and Physics 102(2-3):176-180. doi:10.1016/j.matchemphys.2006.11.022 Abstract In order to develop a better understanding of p-doping in SnO2 nanoparticles, a gel-sol method of synthesis is employed to systematically examine the influence of reaction and growth conditions such as pH, dopant concentration, and calcinations temperature. The extent of Li doping in nanoparticles is characterized using nuclear reaction analysis (NRA) and the nanostructure with high-resolution electron transmission microscopy (HR-TEM) and X-ray diffraction (XRD) techniques. The NRA reports that the lithium incorporation only takes place at alkaline pH (> 8). The XRD and TEM results indicate that the crystallite size of exhibits a maximum at a pH of 8 increasing (decreasing) with calcinations temperature (dopant concentration), while preserving the casserite crystal structure (tetragonal rutile) under these reaction conditions. A preliminary analysis suggests a nanoscopic phase separation of lithium-poor crystalline core and lithium rich amorphous shell may occur with increased dopant concentration.
2007. "On the stability and oxidation of single crystal (100) InAs surfaces." ECS Transactions 11(4):409-419. doi:10.1149/1.2779577 Abstract We report on a method to prepare clean and smooth surfaces of InAs (100) along with in-situ high-resolution studies of the nanoscale oxidation of the pristine surface. A hydrogen molecular cleaning (HMC) technique has been developed that results in complete removal of native oxide. This has been verified in-depth by in-situ nuclear reaction analysis (NRA) using the 16O(d,p)17O reaction and X-ray photoelectron spectroscopy. Further, ion channeling studies have been performed to verify atomically smooth surfaces after post cleaning. We derive kinetic boundaries for oxide formation on cleaned InAs surfaces using NRA measurements.
2007. "In-situ ion scattering and x-ray photoelectron spectroscopy studies of stability and nanoscale oxidation of single crystal (100) InAs." Applied Physics Letters 90(20):, doi:10.1063/1.2740200 Abstract Preparation of clean and smooth surfaces of InAs(100) by hydrogen molecular cleaning (HMC) along with in-situ studies of the nanoscale oxidation of pristine surfaces is studied. Removal of native oxides has been verified in-depth by in-situ nuclear reaction analysis(NRA) using the 16O(d,p)17O reaction and XPS. Further, ion channeling studies have been performed to verify atomically smooth surfaces after post-cleaning. Stability and kinetic boundaries of the cleaned InAs(100) surfaces against oxidation have also been experimentally derived and studied by NRA. These results are important not only to prepare clean surfaces of InAs, but also to understand fundamentals of oxide/III-V semiconductor interfaces.
2007. "Properties of Structurally Excellent N-doped TiO2 Rutile." Chemical Physics 339(1-3):27-35. doi:10.1016/j.chemphys.2007.04.024 Abstract We have used plasma-assisted molecular beam epitaxy to synthesize structurally near-perfect crystalline films of TiO2-xNx rutile for the first time. These materials allow the properties of TiO2-xNx to be elucidated without the interfering effects of oxygen vacancy defects. In the absence of such defects, the extent of N incorporation in the lattice is limited to 2 ± 1 at. % of the anions. Substitutional N (NO) exhibits a -3 formal charge due to charge transfer from shallow-donor interstitial Ti(III), which forms during epitaxial growth. Hybridization between NO and adjacent lattice Ti ions occurs, resulting in new states off the top of the rutile valence band and an apparent band gap reduction of ~ 0.5 eV. It is not yet known if these new states result in mobile electron-hole pair creation upon irradiation, but experiments are planned to answer this important question.
2006. "Study of Hydrogen Stability in Low-k Dielectric Films by Ion Beam Techniques." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):335–338. doi:10.1016/j.nimb.2006.04.022 Abstract With shrinking device geometries into the 0.13 µm technology node, a transition to low-k dielectrics becomes increasingly attractive. Negative bias temperature instability, which is associated with hydrogen migration at elevated temperatures, becomes the main degradation mechanism of concern for conductivity breakdown in semiconductor devices. The possibility of hydrogen release during each of the fabrication process is, therefore, of great interest to the understanding of device reliability. In the current study, various low-k dielectric films were subjected to thermal annealing in N2 ambient at temperatures that are generally used for device fabrication. Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) were used to investigate composition change and hydrogen redistribution of the dielectric films. The results indicate that organosilicate glass, silicon nitride and silicon oxynitride films were stable at temperatures up to 500 °C. In phosphorus doped silicon glass and plasma-enhanced tetraethylorthosilicate films, significant hydrogen release from the surface region was evident after heat treatment in N2 purged environment at 300 °C for 30 min, further hydrogen release is observed as temperature increases.
2006. "Non-linear Damage Accumulation in Au-irradiated SrTiO3." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 251(1):127-132. doi:10.1016/j.nimb.2006.05.018 Abstract Ion-induced damage in strontium titanate (SrTiO3) has been investigated using 1.0 MeV Au ions at 150 K and room temperatures. Rutherford backscattering spectrometry using 2.0 MeV He+ beam was carried out along the <100> direction to study crystalline damage and the accumulation behavior. Damage accumulation has been determined as the relative disorder on the Sr and Ti sublattices at the damage peak as a function of local dose. A disorder accumulation model has been fit to both the Sr and Ti damage accumulation data, and contributions from the amorphous fraction and the crystalline disorder are discussed. The results indicate that defect-stimulated amorphization is the primary amorphization mechanism in SrTiO3. The sigmoidal accumulation behavior in damage accumulation leads to non-linear increase of the width of the damage profile.
2006. "Comparison of Analytical Techniques for Analysis of Arsenic Adsorbed on Carbon." Water Quality Research Journal of Canada 41(2):185-189. Abstract Activated carbon (AC) has been extensively used to remove trace metals, particularly arsenic, from water for a number of years. To date, attempts to quantify directly the concentration of arsenic in activated carbon using non-destructive methods have been limited. High-energy ion beam based particle induced X-ray emission (PIXE) is ideally suited to investigate the issues regarding the quantification of trace metals in solids. In this study, after the adsorption of arsenic on activated carbon, arsenic concentration in granular activated carbon (GAC) and powder activated carbon (PAC) were quantified using PIXE. The PIXE results were compared with atomic absorption spectrometry (AAS) and inductively coupled plasma (ICP) measurements. Some differences are observed between these measurements. The differences are greater in the case of GAC compared to PAC. These differences are mainly due to inhomogeneous structure of GAC and PAC, which includes the variable surface properties such as surface area and pore sizes in each granule or particle. The larger differences are mainly due to the increased particle dimensions of GAC compared to PAC and the nature of the internal pore structure of GAC, which results in different amount of arsenic adsorbed on different granules of GAC or even in different regions of one granule. This inhomogenity of arsenic concentration is clearly visible in the arsenic concentration map generated for a single GAC particle using microbeam PIXE.
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. "Applications of High Energy Ion Beam Techniques in Environmental Science: Investigation Associated with Glass and Ceramic Waste Forms." Journal of Electron Spectroscopy and Related Phenomena 150(2-3):195-207. doi:10.1016/j.elspec.2005.06.010 Abstract High energy ion beam capabilities including Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) have been very effectively used in environmental science to investigate the ion exchange mechanisms in glass waste forms and the effects of irradiation in glass and ceramic waste forms in the past. In this study, RBS and NRA along with SIMNRA simulations were used to monitor the Na depletion and D and 18O uptake in alumina silicate glasses, respectively, after the glass coupons were exposed to aqueous solution. These results show that the formation of a reaction layer and an establishment of a region where diffusion limited ion exchange occur in these glasses during exposure to silica-saturated solutions. Different regions including reaction and diffusion regions were identified on the basis of the depth distributions of these elements. In the case of ceramics, damage accumulation was studied as a function of ion dose at different irradiation temperatures. A sigmoidal dependence of relative disorder on the ion dose was observed. The defect dechanneling factors were calculated for two irradiated regions in SrTiO₃ using the critical angles determined from the angular yield curves. The dependence of defect dechanneling parameter on the incident energy was investigated and it was observed that the generated defects are mostly interstitial atoms and amorphous clusters. Thermal recovery experiments were performed to study the damage recovery processes up to a maximum temperature of 870 K.
2006. "Synthesis of Room-Temperature Ferromagnetic Cr-doped TiO₂(110) Rutile Single Crystals using Ion Implantation." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):198-200. doi:10.1016/j.nimb.2005.08.149 Abstract Ferromagnetic Cr-doped rutile TiO₂ single crystals were synthesized by high-temperature ion implantation. The associated structural, compositional and magnetic properties were studied by x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, proton induced x-ray emission, x-ray diffraction, Cr K- and L-shell near-edge x-ray absorption spectroscopy, and vibrating sample magnetometry. Cr was distributed uniformly to the depth of about 300 nm with an average concentration of ~1 at. %. The samples are semiconducting and ferromagnetic as implanted, with a saturation magnetization of 0.29B/Cr atom at room temperature. Cr is in a formal oxidation state of +3 throughout the implanted region, and no CrO₂ is detected.
2006. "Quantification of Dopant Concentrations in Diluted Magnetic Semiconductors using Ion Beam Techniques." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):402-405. doi:10.1016/j.nimb.2006.04.038 Abstract It has recently been demonstrated that magnetically doped TiO2 and SnO2 show ferromagnetism at room-temperature and Curie temperatures above room temperature. However, accurate knowledge of dopant concentrations is necessary to quantify magnetic moments in these materials. Rutherford Backscattering spectrometry (RBS) is one of the powerful techniques to quantify magnetic transition metal dopant concentrations in these materials. However, in some cases, the interference of RBS signals for different dopants and substrate elements in these materials makes analysis difficult. In this work, we demonstrate that particle induced x-ray emission (PIXE) can be successfully used to quantify the magnetic transition element dopants in several room temperature ferromagnetic materials synthesized using three different synthesis methods: oxygen plasma assisted molecular beam epitaxy, ion implantation and wet chemical methods.
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. "Characterization of Ambient Aerosols in Mexico City during the MCMA-2003 Campaign with Aerosol Mass Spectrometry. Results from the CENICA Supersite." Atmospheric Chemistry and Physics 6:925-946. Abstract An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, while another was deployed in the Aerodyne Mobile Laboratory (AML) during the Mexico City Metropolitan Area field study (MCMA-2003) from March 29-May 4, 2003 to investigate particle concentrations, sources, and processes. This is the first of a series of papers reporting the AMS results from this campaign. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 μm (NR PM1) with high time and size resolution. For the first time, we report field results from a beam width probe, which was used to study the shape and mixing state of the particles and to quantify potential losses of irregular particles due to beam broadening inside the AMS. Data from this probe show that no significant amount of irregular particles was lost due to excessive beam broadening. A comparison of the CENICA and AML AMSs measurements is presented, being the first published intercomparison between two quadrupole AMSs. The speciation, and mass concentrations reported by the two AMSs compared well. In order to account for the refractory material in the aerosol, we also present measurements of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from PIXE analysis of filters. Comparisons of (AMS + BC + soil) mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a Tapered Element Oscillating Microbalance (TEOM) and a DustTrack Aerosol Monitor) are also presented. The comparisons show that the (AMS + BC + soil) mass concentration during MCMC-2003 is a good approximation to the total PM₂․₅ mass concentration.
2006. "Oxidation studies of CrAlON nanolayered coatings on steel plates." Surface & Coatings Technology 201(3-4):1685-1694. doi:10.1016/j.surfcoat.2006.02.053 Abstract The requirements of low cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell stacks have directed attention to the use of metal plates with oxidation resistant coatings. We have investigated the performance of steel plates with nanolayered coatings consisting of [CrON/AlON]n. The coatings were deposited using large-area filtered arc deposition technology, with various O/N pressure ratios, and subsequently annealed in air for up to 25 h at 800 °C. The composition, structure and surface morphology of the coated plates were characterized using RBS, nuclear reaction analysis, and AFM techniques. By altering the architecture and composition of the coatings, the rate of oxidation was reduced relative to the uncoated steel plates, and Fe diffusion from the substrate to the surface through the coating was significantly reduced.
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. "Aerosol Composition and Source Apportionment in the Mexico City Metropolitan Area with PIXE/PESA/STIM and Multivariate Analysis." Atmospheric Chemistry and Physics 6(12):4591-4600. Abstract Aerosols play an important role in the atmosphere but are poorly characterized, particularly in urban areas like the Mexico City Metropolitan Area (MCMA). The chemical composition of urban particles must be known to assess their effects on the environment, and specific particulate emissions sources should be identified to establish ef- 5 fective pollution control standards. For these reasons, samples of particulate matter _2.5 µm (PM2.5) were collected during the MCMA-2003 Field Campaign for elemental and multivariate analyses. Proton-Induced X-ray Emission (PIXE), Proton-Elastic Scattering Analysis (PESA) and Scanning Transmission Ion Microscopy (STIM) techniques were done to determine concentrations of 19 elements from Na to Pb, hydrogen, and 10 total mass, respectively. The most abundant elements from PIXE analysis were S, Si, K, Fe, Ca, and Al, while the major emissions sources associated with these elements were industry, wind-blown soil, and biomass burning. Wind trajectories suggest that metals associated with industrial emissions came from northern areas of the city whereas soil aerosols came from the southwest and increased in concentration during 15 dry conditions. Elemental markers for fuel oil combustion V and Ni correlated with a large SO2 plume to suggest an anthropogenic, rather than volcanic, emissions source. By subtracting major components of soil and sulfates determined by PIXE analysis from STIM total mass measurements, we estimate that approximately 50% of PM2.5 consisted of carbonaceous material.
2006. "Synthesis and Characterization of Cobalt Silicide Films on Silicon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):532–535. doi:10.1016/j.nimb.2006.03.046 Abstract Cobalt silicide has emerged as a leading contact material in silicon technology due to its low resistivity, high stability and small lattice mismatch. In this study, 0.2-0.4 micron thick Co films were deposited on Si(100) wafers by RF magnetron sputtering at room temperature, and annealed at temperatures from 600 °C to 900°C in vacuum. The as-deposited and annealed samples were characterized by Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), x-ray diffraction (XRD) and scanning electron microscopy (SEM). Although the Si substrates were sputter cleaned before the deposition, all the samples showed a thin oxide layer at the Si/Co interfaces. Annealing up to 700 °C did not alter the composition at the interface except small amount Co diffusion into Si. Annealing at 800 °C promotes the evaporation of the oxides from the interface and, as a result, clean CoSi2 films were formed. Although the interface appeared to be sharp within the RBS resolution, the surface topography was relatively rough with varying size of the crystal grains after high temperature annealing.
2006. "Temperature response of 13C atoms in amorphized 6H-SiC." Applied Physics Letters 89(26):art. no.:261902, (3 pages). doi:10.1063/1.2422892 Abstract Implantation of 13C2+ ions was employed to produce a concentration profile in 6H-SiC at 140 K. In-situ study of 13C implanted species was performed using the resonant reaction of 13C(p,gamma)14N at Ep=1.748 MeV. Significant 13C diffusion in the amorphized SiC does not occur up to 1130 K. The presence of Au implants (1.9 at.%) does not affect the 13C behavior. High-energy H+ irradiation also does not promote the 13C diffusion. The results suggest that C atoms are readily trapped locally in the SiC structure during disordering, which is important to understanding the amorphization processes in SiC.
2006. "Hydrogen behavior in Mg+-implanted graphite." Journal of Materials Research 21(4):811-815. doi:10.1557/JMR.2006.0121 Abstract A graphite wafer has been implanted with Mg+ to produce a uniform Mg concentration. Subsequent H+ implantation covered both the Mg+-implanted and unimplanted regions. Ion-beam analysis shows a higher H retention in graphite embedded with Mg than in regions without Mg. A small amount of H diffuses out of the H+ implanted graphite during thermal annealing at temperatures up to 300°C. However, significant H release from the region implanted with both Mg+ and H+ ions occurs at 150°C; further release is also observed at 300°C. The results suggest that there are efficient H trapping centers and fast pathways for H diffusion in the Mg+ implanted graphite, which may prove highly desirable for reversible H storage.implanted graphite, which may prove highly desirable for reversible H storage.
2006. "Tribological performance of hybrid filtered arc-magnetron coatings - Part I: Coating deposition process and basic coating properties characterization." Surface & Coatings Technology 201(6):3732-3747. doi:10.1016/j.surfcoat.2006.09.007 Abstract Aircraft propulsion applications require low-friction and wear resistant surfaces that operate under high contact loads in severe environments. Recent research on supertough and low friction nanocomposite coatings produced with hybrid plasma deposition processes was demonstrated to have a high potential for such demanding applications. However, industrially scalable hybrid plasma technologies are needed for their commercial realization. The Large area Filtered Arc Deposition (LAFAD) process provides atomically smooth coatings at high deposition rates over large surface areas. The LAFAD technology allows functionally graded, multilayer, super-lattice and nanocomposite architectures of multi-elemental coatings via electro-magnetic mixing of two plasma flows composed of different metal ion vapors. Further advancement can be realized through a combinatorial process using a hybrid filtered arc-magnetron deposition system. In the present study, multilayer and nanostructured TiCrCN/TiCr +TiBC composite cermet coatings were deposited by the hybrid filtered arc-magnetron process. Filtered plasma streams from arc evaporated Ti and Cr targets, and two unbalanced magnetron sputtered B4C targets, were directed to the substrates in the presence of reactive gases. A multiphase nanocomposite coating architecture was designed to provide the optimal combination of corrosion and wear resistance of advanced steels (Pyrowear 675) used in aerospace bearing and gear applications. Coatings were characterized using SEM/EDS, XPS and RBS for morphology and chemistry, XRD and TEM for structural analyses, wafer curvature and nanoindentation for stress and mechanical properties, and Rockwell and scratch indentions for adhesion. Coating properties were evaluated for a variety of coating architectures. Thermodynamic modeling was used for estimation of phase composition of the top TiBC coating segment. Correlations between coating chemistry, structure and mechanical properties are discussed.
2006. "Irradiated microstructure of alloy 800H ." Journal of Nuclear Materials 351(1-3):223-227. doi:10.1016/j.jnucmat.2006.02.009 Abstract Austenitic alloy 800H has the same basic composition as INCOLOY alloy 800 (Fe–20Cr–32Ni) but with significantly higher creep-rupture strength due to a required 60 lm minimum grain size. It is one of the high temperature candidate alloys being considered for Generation IV nuclear reactor systems. The radiation resistance of 800H has not been previously studied. This work provides information on the microstructural changes in 800H after irradiation using 5.0 MeV Ni ions at 500 *C to 5 and 50 dpa. Following irradiation, changes in microstructure and phase stability were studied using transmission electron microscopy (TEM). At a dose of 50 dpa, no voids were found and the density and size of the faulted loops were measured to be 2.3 · 1016 cm*3 and 8.4 nm, respectively. There are fine precipitates distributed in 800H with an average size approximately 6 nm and a density greater than 9.1 · 1015 cm*3. The high Ni content and the presence of precipitates are believed to be responsible for the resistance to void formation at dose up to 50 dpa.
2006. "Microstructural development in advanced ferritic–martensitic steel HCM12A." Journal of Nuclear Materials 351(1-3):174-186. doi:10.1016/j.jnucmat.2006.02.014 Abstract HCM12A is an advanced nominal 12Cr ferritic–martensitic steel designed for higher temperature operation and is under consideration for application in core components in Generation IV nuclear energy systems. This work provides information on the hardening and microstructural changes in HCM12A after irradiation using 2.0 MeV protons at 400 *C to 10 dpa and at 500 *C to 3 dpa, and using 5 MeV Ni-ions at 500 *C to 50 dpa. Following irradiation, changes in hardness were measured using Vickers hardness indentation, changes in microstructure and phase stability were studied using transmission electron microscopy, and changes in microchemistry were measured using scanning Auger microscopy and analytical electron microscopy. The hardness at 400 *C increases by roughly 70% and saturates by roughly 5 dpa. The microstructural changes contributing to this hardness increase are mainly the formation of precipitate phases. Hardness increases are much smaller at 500 *C. Chromium is enriched at grain boundaries prior to irradiation, likely due to grain boundary carbides, and increases further during irradiation at least partially due to radiation-induced segregation.
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. "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. "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. "Characterization of Ambient Aerosols in Mexico City during the MCMA-2003 Campaign with Aerosol Mass Spectrometry. Part II: Overview of the Results at the CENICA Supersite and Comparison to Previous Studies." Atmospheric Chemistry and Physics Discussions 5:4183-4221. Abstract An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite during the Mexico City Metropolitan Area field study from March 29-May 4, 2003. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 μm (NR PM1) with high time and size resolution. Measurements of Black Carbon (BC) using an aethalometer, and estimated soil concentrations from Proton-Induced X-Ray Emission (PIXE) analysis of impactor substrates are also presented and combined with the AMS in order to include refractory material and estimate the total PM₂․₅ mass concentration at CENICA during this campaign. In Mexico City, the organic fraction of the estimated PM₂․₅ at CENICA represents 55% of the mass, with the rest consisting of inorganic (mainly ammonium nitrate and sulfate/ammonium salts) compounds, BC, and soil. Inorganic compounds represents 27.5% of PM₂․₅; BC mass concentration is equivalent to about 11%; while soil represents about 7%. The NR species and BC have diurnal cycles that can be qualitatively interpreted as the interplay of direct emissions, photochemical production in the atmosphere followed by condensation and gas-to-particle partitioning, boundary layer dynamics, and/or advection. Bi and trimodal size distributions are observed, with a small primary organic combustion (likely traffic) particle mode and an accumulation mode that contains mainly organic and secondary inorganic compounds. The AMS and BC mass concentrations, size distributions, and diurnal cycles are found to be qualitatively similar to those from other field measurements in Mexico City.
2005. "Ferromagnetic Cr-doped ZnO for Spin Electronics via Magnetron Sputtering." Journal of Applied Physics 97:10D310. doi:10.1063/1.1847914 Abstract Through nonequilibrium rf magnetron co-sputtering of ZnO and Cr metal on a-plane Al₂O₃ we find ferromagnetic ordering with a room-temperature saturation moment of 1.4 mB per Cr ion at a doping concentration of ,9.5 at. % after UHV postanneal. No secondary phases are detected in the films via x-ray diffraction sXRDd. Increased Cr doping causes disorder within the films resulting in decreased overall, and per Cr, moment. The Curie temperature exceeds 365 K, the maximum temperature reached in our experiment. All films are dielectric with a resistivity higher than 10₆ V cm at room temperature. The lack of carriers indicates that the ferromagnetic mechanism is not carrier mediated.
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. "Near-Edge X-ray Absorption Fine-Structure Study of Ion-Beam-Induced Phase Transformation in Gd2(Ti1-yZry)2O7." Journal of Applied Physics 97(3):Article 033518. Abstract The structural and electronic properties of Gd2(Ti1-yZry)2O7 (y=0-1) pyrochlores following 2.0 MeV Au2+ ion-beam irradiation (~5.0x1014 Au2+/cm2) have been investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure (NEXAFS). The irradiation of Gd2(Ti1-yZry)2O7 leads to the phase transformation from the ordered pyrochlore structure (Fd3m) to the defect fluorite structure (Fm3m) regardless of Zr concentration. Irradiated Gd2(Ti1-yZry)2O7 with y≤0.5 are amorphous although significant short-range order is present. Contrasting to this behavior, compositions with y≥0.75 retain crystallinity in the defect fluorite structure following irradiation. The local structures of Zr4+ in the irradiated Gd2(Ti1-yZry)2O7 with y≥0.75 determined by NEXAFS are the same as in cubic fluorite-structured yttria-stabilized zirconia (Y-ZrO2), thereby providing conclusive evidence for the phase transformation. The TiO6 octahedra present in Gd2(Ti1-yZry)2O7 are completely modified by ion-beam irradiation to TiOx polyhedra, and the Ti coordination is increased to eight with longer Ti-O bond distances. The similarity between cation sites and the degree of disorder in Gd2Zr2O7 facilitate the rearrangement and relaxation of Gd, Zr, and O ions/defects. This inhibits amorphization during the ion-beam induced phase transition to radiation-resistant defect fluorite structure contrasting to the ordered Gd2Ti2O7. Thus, during the ion-beam induced phase transition, the Gd2(Ti1-yZry)2O7 with y≤0.5 becomes amorphous whereas compositions with y≥0.75 retain crystalline structure.
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. "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. "Erratum to: “Carbon analysis using energetic ion beams” [Nucl. Instr. and Meth. B 222 (2004) 538-546] ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 227(3):450-451. Abstract This paper corrects the energy calibration for the accelerator system we used for carbon cross section measurements.
2005. "Cr-doped TiO₂ Anatase- A Ferromagnetic Insulator ." Journal of Applied Physics 97:046103. Abstract Epitaxial ferromagnetic films of Cr-doped TiO₂ anatase (CrxTi₁-xO₂-x/₂) were grown on LaA1O3(001) using oxygen-plasma assisted molecular beam epitaxy. Cr K-shell x-ray absorption near-edge spectroscopy shows that the formal oxidation state of Cr is +3 throughout the films with no evidence for either elemental Cr or half-metallic CrO₂. Cr is found to substitute for Ti in the lattice with uniform distribution throughout the doped region of the film. The Cr-doped anatase films exhibit room temperature ferromagnetism aligned in-plane with a saturation magnetization of ~0.6 µB/Cr atom.
2005. "Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia." Applied Physics Letters 86(13):131906-131909. Abstract Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.
2005. "The Stability of 9Cr-ODS Oxide Particles Under Heavy-Ion Irradiation ." Nuclear Science and Engineering 151(3):305-312. Abstract An oxide-dispersion-strengthened (ODS) martensitic steel 9Cr-ODS was irradiated with 5-MeV Ni ions at 500°C at a dose rate of 1.4 × 10-3 dpa/s to doses of 5, 50, and 150 dpa. The ODS steel has been designed for use in higher-temperature energy systems. However, the radiation effects are not fully characterized, particularly to high doses. Dense dislocations, precipitates, and yttrium-titanium oxide particles dominated the microstructure of 9Cr-ODS for both the unirradiated and irradiated cases with no dislocation loops observed. No voids were detected for doses up to 150 dpa. The average size of the oxide particles, whose size is approximately described by a lognormal distribution, slightly decreased with dose from ~12 nm for the unirradiated case to ~9 nm at 150 dpa. The decrease in size follows a square root of dose dependency, indicating the effect is radiation induced. The decrease in size is not expected to have a detrimental effect on high-temperature strength, even to extremely high dose.
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. "Damage Evolution on Sm and O Sublattices in Au-Implanted Samarium Titanate Pyrochlore." Journal of Applied Physics 95(5):2866-2872. Abstract Damage evolution on the Sm and O sublattices in Sm₂Ti₂O₇ single crystals irradiated with 1 MeV Au₂⁺ ions at 170, 300 and 700 K was studied by Rutherford backscattering spectroscopy and 16O(d,p)17O nuclear reaction analysis. The damage accumulation behavior at each irradiation temperature indicates that the relative disorder on the O sublattice is higher than that on the Sm sublattice, and the relative disorder on each sublattice follows a nonlinear dependence on dose that is well described by a disorder accumulation model. While there is little difference in damage accumulation behavior on the Sm sublattice at 170 and 300 K irradiation, the rate of damage accumulation decreases dramatically at 700 K due to dynamic recovery. The critical dose for amorphization at 170 and 300 K is ~0.14 dpa, and a higher dose of ~ 0.22 dpa is observed under irradiation at 700 K. During thermal annealing in an 18O environment, a significant increase in the 18O exchange was observed between 800 and 900 K, which is just below the previously determined critical temperature, 950 K, for amorphization in Sm₂Ti₂O₇, suggesting that the mobility of O vacancies may be important in defining the critical temperature.
2004. "Damage Accumulation and Amorphization in Samarium Titanate Pyrochlore." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:89-94. Abstract Damage accumulation in Sm2Ti₂O₇ single crystals irradiated with Au₂⁺ ions at 170, 300 and 700 K was studied by Rutherford backscattering spectrometry using a 2.0 MeV He⁺ beam along the <001> channeling direction. The relative disorder on the Sm sublattice follows a nonlinear dependence on ion fluence. The nonlinear behavior is described well by a disorder accumulation model that indicates a predominant role of a defect-stimulated amorphization process. The critical dose for amorphization at 300 K is ~0.14 dpa, which is in good agreement with in-situ transmission electron microscopy results for polycrystalline Sm₂Ti₂O₇ irradiated with 600 keV Bi⁺ ions and with Gd₂Ti₂O₇ doped with 244Cm. Despite the six orders of magnitude difference in damage rates, the good agreement between the amorphization doses in Sm2Ti₂O₇ at 300 K and 244Cm-doped Gd₂Ti₂O₇ at 340 K indicates that damage accumulation at these temperatures is relatively independent of dose rate.
2004. "Annealing Behavior of Al-Implantation-Induced Disorder in 4H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:647-651. Abstract Single crystal 4H-SiC films were implanted at 150 K with 1.1 MeV Al₂₂⁺ and subsequently annealed at elevated temperatures. Rutherford backscattering spectrometry (RBS) results indicate that the relative Si disorder at the damage peak recovers significantly as the annealing temperature increases. However, the residual Si disorder is more resistant to high-temperature annealing in the region of the implanted Al. The maximum concentration of Al profile measured by secondary ion mass spectroscopy (SIMS) is a factor of 1000 lower than the level of the residual Si disorder at the same region. Analysis of these results indicates that the excess residual Si disorder around the implanted Al projected range cannot be accounted for by just the Al interstitials; instead, it appears that each implanted Al stabilizes or inhibits recovery for an equivalent of a few hundred Si interstitials under the current experimental conditions.
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. "Epitaxial Growth and Properties of Cobalt-doped ZnO on α-Al₂O₃ Single-Crystal Substrates." Physical Review. B, Condensed Matter 70(5):054424. Abstract Co-doped ZnO (CoxZn₁-xO) is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial CoxZn₁-xO films on Al₂O₃(012) substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of Co (x < 0.35) can be uniformly incorporated into the film without phase segregation. Co is found to be in the ⁺² oxidation state, independent of x. This material can be grown n type by the deliberate incorporation of oxygen vacancies, but not by inclusion of ~1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small (< 0.1 B/Co), even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the Co(II) ions behave as magnetically isolated S = 3/2 spins.
2004. "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. "Using CrAIN Multilayer Coatings to Improve Oxidation Resistance of Steel Interconnects for Solid Oxide Fuel Cell Stacks." Journal of Materials Engineering and Performance 13(3):295-302. Abstract The requirements of low cost and high-tempurature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell stacks has directed attention to the use of metal plates with oxidation resistant coatings. We have investigatedt he performance of steel plates with multilayer coatings consisting of CrN for electrical conductivity and CrAIN for oxidation resistance. The coatings were deposited usin large area filterd arc deposition technolgy, and subsequently annealed in air for up to 25 hours at 800 degrees celsius. The composition, structer and morphology of the coated plates were characterized using RBS, nuclear reaction analysis, AFM and TEM techniques. By altering the architecture of the layers within the coatings, the rate of oxidation was reduced by more than an order of magnitute. Electrical resistance was measured at room temperature.
2004. "NRA and ERDA Investigation of Helium Retention in SiC as a Function of Irradiation and Annealing." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:631-635. Abstract Silicon carbide has been proposed for coating applications in advanced reactor designs, so studies of its behavior in the presence of ion irradiation and fission products are of Interest. We investigated the retention of He in single crystal 6H SiC as a function of irradiation dose and annealing temperature using both nuclear reaction analysis (NRA) and time-of-flight elastic recoil detection analysis (ToF ERDA). Ions of 3He⁺ were implanted at 40 keV in SiC to a depth of ~200 nm at room temperature. NRA was performed using 1.0 MeV D⁺ and the 3He(D,α)1H reaction. No change in the He profile was seen for irradiation dose up to 6.8x1017 D⁺/cm₂ at room temperature. Isochronal annealing of the SiC between 300 and 1200 K also showed no significant helium loss. Subsequently, a sample was irradiated with D⁺ at 900 K and again at 1100 K. No loss of 3He associated with irradiation was seen for a dose up to 5x1017 D⁺/cm₂. Annealing the sample above 1200 K resulted in thermally activated loss of He. ToF ERDA measurements were performed using 44 MeV 127I10⁺ for both irradiation and analysis. Depth profiles of the He distribution showed no significant change under I bombardment with an ion fluence up to ~1014/cm₂ at room temperature. NRA was performed on the implanted sample subjected to ERDA. The 3He profiles for regions subjected to I irradiation were similar in shape to those with no I irradiation.
2004. "Room-Temperature Ferromagnetism in Ion-Implanted Co-Doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Interest in diluted magnetic semiconductros (DMS) is growing rapidly within the emerging field of spintronics. For example, the ability to efficiently inject spin-polarized carriers into nonmagnetic semiconductor heterostructures creates new and exciting possibilities for utilizing DMS materials in spin-based devices. Several III-V and II-VI semiconductor materials doped with magnetic transition metal elements have been explored. Although these materials show promising behavior in some cases, most exhibit Curie temperatures of ~170 K or less. It has recently been shown that certain oxide semiconductors doped with magnetic transition elements show room-temperature ferromagnetism.
2004. "Room Temperature Ferromagnetism in Ion-implanted Co-doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Ferromagnetic Co-doped rutile TiO₂ singel crystals were successfully synthesized by high temperature ion implantation and charaterized by a variety of techniques. Co is uniformly distrubuted to the depth of ~300nm with an average concentration of ~2 at. %, except in the near-surface region, where the concentration is ~3 at. %. Magnetic measurements reval ferromagnetic behavior at room temperature with an effective saturation magnetization of ~0.6 ub/Co atom. The Co formal oxidation state is found to be +2 throughouth the implanted region, and no Co(0) is detected.
2004. "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. "Room-Temperature Ferromagnetism in Chemically Synthesized Sn₁-xCox O₂ Powders." Applied Physics Letters 85(9):1559-1561. Abstract Room temperature ferromagnetism is observed in chemically synthesized powder samples of Sn₁₋xCox O₂ with x = 0.005 and 0.01. Magnetic hysteresis loops are ovserved at 300K with coercivity Hc ~ 630 Oe, saturation magnetization Ms ~0.133μв and about 31% remenance. Analyses of the magnetization data of paramagnetic samples with x = 0.01 and 0.03, measured as a function of temperature (3-330K) and magnetic field (up to 50kOe), indicated the presence of Co²⁺ ions with spin S = 3/2. Magnetic data obtained from samples prepared at different temperatures indicate that the observed ferromagnetism for x ≤ 0.01 might have been triggered by changes in the oxygen stiochiometry.
2004. "Studies of Two-and Three-Dimensional ZnO:Co Structures Through Different Sythetic Routes." Journal of Applied Physics 95(11):7393-7395 Part 2. Abstract Multilayers of Co and ZnO, with nominal layer thicknesses on the atomic scale with up to 25 bilayers, were deposited by ion beam sputtering on silicon and glass substrates at ambient temperature. Thick epitaxial CoxZn₁-xO films on A1₂O₃(012) substrates were grown by metalorganic chemical vapor deposition using a liquid precursor delivery system. All were co-doped with A₁. Comparative analysis of magnetization, resistivity, and magnetoresistance measurements, performed in the temperature range 2.5-300K, is presented. At small thickness of Co layers in the multilayer samples, these structures are diluted magnetic semiconductor (DMS) superlattices, with properties close to the epitaxial films. A crossover from DMS to discontinuous magnetic metal/semiconductor multilayers is observed with increasing metal content in the multilayers. This leads to changes in conduction mechanisms, with increasing contribution of quasi-three-dimensional or quasi-two-dimensional inter-granular hopping, and superparamagnetism.
2004. "Probing Cation Antisite Disorder in Gd₂Ti₂O₇ Pyrochlore by Site-Specific Near-Edge X-ray-Absorption Fine Structure and X-ray Photoelectron Spectroscopy." Physical Review. B, Condensed Matter and Materials Physics 70(10):100101(R), 1-4. Abstract Disorder in Gd₂Ti₂O₇ is investigated by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS). NEXAFS shows Ti⁺⁴ ions occupy octahedral sites with a tetragonal distortion induced by vacant oxygen sites. O 1s XPS spectra obtained with a charge neutralization system from Gd₂Ti₂O₇ (100) and the Gd₂Ti₂O₇ pyrochlore used in Phys. Rev. Lett. 88, 105901 (2002), both yielded a single peak, unlike the previous result on the latter that found two peaks. The current results give no evidence for an anisotropic distribution of Ti and O. The extra features reported in the aforementioned communication resulted from charging effects and incomplete surface cleaning. Thus, a result confirming the direct observation of simultaneous cation–anion antisite disordering and lending credence to the split vacancy model has been clarified.
2004. "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. "Thermal and Dynamic Responses of Ag Implants in Silicon Carbide ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:642-646. Abstract A single crystal wafer of 6H-SiC was sequentially implanted in two different areas at 210 and 873 K, respectively, to the same fluence of 500 Ag₂⁺/nm₂. Rutherford backscattering spectrometry (RBS) has been used in a random orientation to profile the Ag implants and along the <0001>-axial channeling direction to determine the defect concentrations. Additional irradiation at 873 K with 5.4 MeV Si₂⁺ ions does not promote diffusion of the implants in either the crystalline or fully-amorphized SiC. There is no evidence of significant diffusion of the implanted Ag in crystalline SiC during thermal annealing at temperatures up to 1573 K. However, it appears that the Ag tends to diffuse toward the surface in amorphous SiC at 1573 K.
2004. "Carbon Analysis using Energetic Ion Beams." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 222(3-4):538-546. Abstract Both Nuclear reaction analysis and non-Rutherford elastic scattering have been widely used for analysis of light elements in solids; these two ion-beam methods complement more traditional analysis by Rutherford backscattering spectrometry. In this study, the reaction/scattering cross-sections for 12C(p,p)12C, 12C(d,p)13C, and 12C(a,a)12C at an angle of 150 (degrees) are measured over relevant energy regions using thin films of carbon (5.8 ug/cm2) on silicate glass. The results are plotted and tabulated as a function of ion energy, with typical uncertainties of 4% for the cross section data. In addition, the angular distribution of the cross sections for the reaction 12C(d,p)13C at an ion energy of 0.94 MeV has also been determined from 100 (degrees) to 170 (degrees). An example for the application of this reaction to SiC is given.
2004. "High Temperature Oxidation Resistance and Surface Electrical Conductivity of Stainless Steels with Filtered Arc Cr-Al-N Multilayer and/or Superlattice Coatings." Surface & Coatings Technology 188-189:55-61. Abstract The requirements for low cost and high-tempurater corrosion resistance for bipolar interconnect plates in solid oxide fuel cell (SOFC) stacks has directed attention to the use of metal plates with oxidation resistant coatings. Candidate coatings must exhibit chemical and thermal-mechanical stability and high electrical conductivity during long-term (>400,000 hrs) exposure to SOFC operatong conditions. The high temperature oxidation resistance and surface electrical donductivity of 304, 440A,a dn Crofer-22 APU steel coupons, with and without multilayer and/or superlattice coatings from a Cr-Al-N system were investigated as a function of exposure in an oxidization atmosphere at high temperatures. The coatins were deposited using large area filtered arc depsition (LAFAD) technology [1], and subsequently annealed in air at 800 degrees C for varying times. Area specific resistance and activation energy for electrical conductivity of oxidized coupons were measured using a 4-point technique with Pt paste for electrical contact between facing oxidized coupon surfaces. The surface compositon, structure and morphology of the coupons were characterized using RBS, nuclear reaction analysis, XPS, SEM, and AFM techniques. The structure of the CRN/CrAlN multilayered superlattice coatings was characterized by TEM. By altering the architecture of the coating layers, both surface electrical conductivity and oxidation resistance [2] improved signigicantly for some of the coated samples tested up to ~100hrs.
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. "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. "Accumulation of Ion Beam Induced Disorder in Stronium Titanate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 206:162-165. Abstract Damage accumulation has been investigated in single crystal SrTiO₃ (100) irradiated with 1.0 MeV Au₂þ by using in situ Rutherford backscattering spectrometry in channeling geometry (RBS/C). Samples were irradiated at temperatures of 170, 300 and 350 K with ion .uences ranging from 0.05 to 0.60 Au₂þ/nm₂. The in situ RBS/C analysis indicates that the relative disorder in both Sr and Ti sublattices shows a strong sigmoidal dependence on ion dose. After an ion .uence of 0.30 Au₂þ/nm₂ at 170 K, the buried region at the damage peak (_60 nm) becomes fully amorphous, which corresponds to a dose of _0.39 dpa. For irradiation at 300 and 350 K, ion .uences of 0.40 Au₂þ/nm₂ (_0.52 dpa) and 0.45 Au₂þ/nm₂ (_0.59 dpa) are necessary to achieve an amorphous state at the damage peak, respectively.
2003. "Energy and mass dependence of isotopic enrichment in sputtering." Applied Physics A, Materials Science and Processing 76(7):1093-1097. Abstract When a solid surface containg more thatn on component is bombarded by energetic particles, the sputtered flux is found to deviated fromt he stoichiometric composition of the target. Thisis known as preferential sputtering. Usually the sputtered flux is enriched with the lighter-mass particles, particularly at small emission angles. As the bombardment of the target is continued, the target surface becomes depleted in the particles that are preferentially emitted and a steady state is eventually established, where the ratio of the sputtered particles becomes equal to the natural abundance ratio of the particles in the target.
2003. "Investigation of Copper(I) Oxide Quantum Dots by Near-Edge X-ray Absorption Fine Structure Spectroscopy." Chemistry of Materials 20(15):3939-3946. Abstract Copper oxide quantum dots (CuOQD) were grown in various thicknesses on different SrTiO₃(001) surfaces and were investigated by near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The experimental growth conditions for the CuOQD were optimized to obtain Cu₂O as the major phase. The CuOQD grown on clean SrTiO₃(001) surfaces at 825 K or higher with p(O₂) of 9.0x10-7 Torr or above contain mostly CuO contrasting to CuOQD grown at 800 K with p(O₂) of ~7.0x10-7 Torr that contain primarily Cu₂O. Furthermore, it is established that there is a strong interaction between the SrTiO₃(001) surface and the first few monolayers of the CuOQD, which induces the formation of Cu(II). However, this interaction is mitigated with increasing thickness of CuOQD resulting in the exclusive formation of Cu₂O in the topmost layers. The influence of the SrTiO₃(001) substrate on the formation of CuOQD can be reduced by modifying the substrate surface using chemical treatment and/or energetic Au₂⁺ ion-beam irradiation, since the substrate effect results from the reaction between the substrate oxygen and the copper atoms from the CuOQD. Examination of the photochemical properties of these CuOQD shows that prolonged soft x-ray irradiation under vacuum reduces Cu(II), which is present as a minor impurity in the CuOQD.
2003. "Oxygen Analysis Using Energetic Ion Beams." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(4):453-461. Abstract Using a thin amorphous layer of SiO₂ (5.2 mg/cm2) on Si, cross sections for the nuclear reactions 16O(d,p1)17O, 16O(d,a)14N and 16O(a,a)16O at a laboratory angle of 150° are determined over energies ranging from 0.70 to 1.06 MeV for D⁺ ions and from 2.95 to 3.05 MeV for He⁺ ions. The results are plotted and tabulated as a function of ion energy. An example for the analysis of atomic displacements on the O sublattice in a Au₂⁺-irradiated SrTiO₃ single crystal is given
2003. "Ion Beam Analysis of Irradiation Effects in 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):92-99. Abstract Irradiation in 6H-SiC single crystal wafers has been performed at temperatures ranging from 150 to 550 K using 2.0 MeV Au₂⁺ or at 300 K using 50 keV He⁺ ions. Additional irradiation for the He⁺-irradiated specimen was carried out near room temperature using 50 MeV I10⁺ ions to ~0.1 ions/nm₂. In-situ isothermal annealing for 6H-SiC irradiated at 500 K to 2.0 Au₂⁺/nm₂ was also conducted up to 90 min at the irradiation temperature. The lattice disorder in the irradiated samples has been determined using either 2.0 MeV He⁺ or 0.94 MeV D⁺ channeling analysis along the <0001> axis. Results show that there is a substantial diffusion of the Si defects into a greater depth during the Au₂⁺ irradiation at 500 and 550 K. Complete amorphization at 550 K does not occur up to a maximum fluence of 15 Au₂⁺/nm₂ in this study. Significant thermal recovery of the Si defects produced at 150 K was not observed during the subsequent thermal annealing at 500 K. Following the I10⁺ irradiation in the He⁺-irradiated specimen near room temperature, remarkable recrystallization at the amorphous-crystalline interfaces around the damage profile is observed.
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. "High Energy Ion Beam Studies of Ion Exchange in a Na₂O-Al₂O₃-SiO₂ Glass." Journal of Applied Physics 91(4):1910-1920. Abstract As part of understanding the processes leading to sodium release and ion exchange, the surface and near surface reaction regions on several specimens of a Na₂O-Al₂O₃-SiO₂ glass have been examined after exposures to isotopicaly labeled aqueous solutions. The majority of the analyses describe here have been carried out using energetic ion beam analysis. Rutherford backscattering spectrometry (RBS) has been used to measure the overall glass composition and to determine the profiles and amounts of Na released from the surface. An important part of the ion exchange process is the uptake and incorporation of hydrogen and oxygen in the glass from the solution. To facilitate this analysis, the glasses were exposed to a solution containing 18O and deuterium and analyzed by accelerator based nuclear reaction analysis (NRA). To confirm some of the RBS depth profile data very near the surface, XPS depth profiles were collected on some samples. Although the Na concentration is decreased in the near surface region, it is not totally removed from the outer surface. In this same region, there is also a significant amount of 18O incorporated demonsrating considerable interaction between the water and the glass. Deeper into the material the amounts of deuterium and 18O are more consistent with water or H3O+ diffusion. These results suggest that there exist an outer reaction layer and an inner diffusion controlled layer in the surface region of the reacted glass.
2002. "Direct Observation of Atomic Disordering at the SrTiO3/Si Interface Due to Oxygen Diffusion." Applied Physics Letters 80(10):1803-1805. Abstract The stability of epitaxially grown single crystal SrTiO3(001) thin films on Si(100) substrate was studied as a function of temperature under vacuum and oxygen rich environments using Rutherford backscattering spectrometry (RBS) along with channeling techniques, nuclear reaction analysis (NRA), and x-ray photoelectron spectroscopy (XPS). During the vacuum annealing, it was found that the interfacial silica formed due to diffusion of oxygen from the film to Si. This was further accompanied by the atomic disordering of Sr, Ti and O sublattices in the film due to its reduction. Although the interfacial process is similar during the heating in oxygen environment, no disordering of the film was observed due to the oxygen replenishment from the environment to the film. Activation energy for the formation of silica at the interface under vacuum heating conditions was determined to be 0.13eV.
2002. "Development of PIXE, PESA and Transmission Ion Microscopy Capability to Measure Aerosols by Size and Time." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 189:284-288. Abstract The elemental analysis of aerosol composition with high time and spatial resolution is crucial in the studies related to environmental issues such as human health, urban smog formation, regional visibility, and climate change. The effects of atmospheric aerosols are closely related to their size distribution, which plays a major role in understanding transport and removal processes and in pinpointing possible aerosol sources. Hence, there is a need for simultaneous measurements of compositions and particle size distribution of aerosols. We have developed a capability that consists of a combination of PIXE, PESA and STIM (same location on the sample) at the accelorator facility in Environmental Molecular Sciences Laboratory (EMSL) to address some of the needs associated with time series and size distribution. Simultaneous measurements of PIXE and PESA can be performed on aerosols collected using 3 stage improved rotating drum impactor by size (3 modes, 2.5 to 0.07 um) and time (2 mm rotation for every 8 hours) on a 20 cm long Teflon strips with a time resolution of 2 hours (using 500 micron size proton beam). Two Teflon strips can be mounted on the manipulator at the same time without breaking the vacuum through a load-lock. Movable and fixed surface barrier detectors are used for PESA and STIM measurements respectively. Preliminary measurements were carried out using the aerosol samples collected at the 62nd floor of Williams Tower in Houston, Texas. These aerosol samples were also analyzed by synchrotron x-ray microprobe (S-XRF) at Advanced Light Source (ALS) and the comparison of XRF and ion beam results along with the details of the capability will be discussed.
2002. "Interfacial Chemistry and the Performance of Bromine-etched CdZnTe Radiation Detector Devices." IEEE Transactions on Nuclear Science 49(4):2005-2009. Abstract The interfacial chemistry and composition of Pt electrodes sputter deposited on bromine-etched CdZnTe surfaces was studied by XPS, SIMS, AES, NRA and RBS. The interfacial composition of a functioning and a non-functioning CdZnTe detector shows significant differences. The degree of cation out-diffusion into the Pt overlayer and the in-diffusion of Pt into the CdZnTe correlate with the degree of oxidation found at the metal-semiconductor interface. Practically all the oxide present at the interface was found to be TeO₂. The results suggest that the inter-diffusion of the atoms and associated charges contribute to stoichiometric variations at the metal-semiconductor interface and influence the electrical performance of the devices.
2002. "Growth and Structure of Epitaxial Ce₁-xZrxO₂ Thin Films on Yttria-Stabilized Zirconia (111)." Journal of Electron Spectroscopy and Related Phenomena 126(1-3):177-190. Abstract We describe here studies aimed at the identification of optimum parameters for the epitaxial growth of the mixed-oxides films, Ce₁ xZrxO₂ with x = 0.1, 0.2 and 0.3, by oxygen-plasma-assisted MBE on single crystal Y-stabilized ZrO₂ (YSZ) substrates. The resulting films were characterized by RHEED, LEED, XPS/XPD, XRD, and RBS/C in order to determine their bulk and surface structures and compositions. Pure-phase, epitaxial Ce₁ xZrxO₂ films readily grew on YSZ(111) without showing any contamination of yttria from the substrate. The resulting epitaxial film surfaces are unreconstructed and exhibit the structure of bulk CeO2(111). XPS data indicate that both Ce and Zr cations are formally in the ⁺⁴ oxidation state for all films prepared here. Small differences in the photoemission results for Zr-doped ceria films as compared to those obtained for pure ZrO₂ may be explained by changes in electronic structure when Zr is added to ceria that, in turn, results from longer Zr O bond distances in the mixed oxides. The minimum yields obtained from the random and channeling spectra of these films also provide evidence that high quality single crystal CeO₂ and Ce₀․₇Zr₀․₃O₂ materials were grown. For the Zr-doped films, Zr atoms are shown to occupy the lattice sites of Ce in the bulk structure of CeO₂ (111). Indeed, based on minimum yield values, the fraction of Zr substitution for Ce cations in the film was estimated to be 88%.
2002. "Deuterium Channeling Study of Disorder in Al-₂(₂⁺)-Implanted 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 190(1-4):636-640. Abstract Single crystal 6H-SiC wafers have been irradiated 60? off normal at 150, 190, 250 and 295 K using 1.1 MeV Al₂₂⁺ ions over fluences from 0.15 to 2.85 ions/nm₂. The accumulation and recovery of disorder on both the Si and C sublattices have been measured simultaneously using in situ 0.94 MeV D⁺ Rutherford backscattering 28Si(d,d)28Si and nuclear reaction 12C(d,p)13C along the <0001>-axial channeling direction. The behavior of disorder accumulation and recovery on the Si and C sublattices is similar. The data suggest that a dynamic recovery stage occurs between irradiation temperatures of 190 and 250 K. At intermediate doses, isochronal annealing (20 min) results show that significant thermal recovery occurs between 420 and 720 K. Complete recovery is not observed by thermal annealing up to the highest temperature (870 K) used in this study.
2002. "Channeling Study of Lattice Disorder and Gold Implants in Gallium Nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 191(1-4):509-513. Abstract Irradiation experiments have been performed 60? off normal for a GaN single crystal film at 300 K using 3 MeV Au3+ ions over fluences ranging from 0.88 to 86.2 ions/nm2. The accumulation of disorder on both the Ga and N sublattices has been simultaneously investigated using 3.8 MeV He+ non-Rutherford backscattering spectrometry along the <0001> and <10 1> axial channeling directions. The accumulated disorder at the damage peak increases with dose below 10 dpa, and saturates at a relative level of ~0.7 between 10 and 60 dpa. Complete amorphization starts at the surface and grows into the damage peak regime. A higher rate of disordering on the N sublattice is observed at low damage levels, which suggests a lower threshold displacement energy on the N sublattice in GaN. Isochronal annealing (20 min) at temperatures up to 1000 K has been used to follow the thermal response of the Ga disorder and Au implants. Some disorder recovery occurs at the intermediate doses. A fraction of Au occupancy on the Ga lattice site is observed in the as-implanted GaN, and the substitutional fraction of the implanted Au increases with increasing temperature.
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.
2001. "Accumulation and Thermal Recovery of Disorder in Au₂⁺-Irradiated SrTiO₃." Journal of Nuclear Materials 289(1-2):204-209. Abstract Damage accumulation and thermal recovery processes have been investigated in single crystal SrTiO₃ (100) irradiated with 1.0 MeV Au₂⁺ using in-situ Rutherford Backscattering Spectroscopy in channeling geometry (RBS/C). Samples were irradiated at temperatures of 170 K and 300 K with ion fluences ranging from 0.10-0.40 Au₂⁺/nm₂. The in situ RBS/C analysis indicates that the relative disorder shows a strong sigmoidal dependence on ion dose. After an ion fluence of 0.30 Au₂⁺/nm₂ at 170 K, the buried region at the damage peak (~60 nm) becomes fully amorphous, which corresponds to a dose of ~ 0.8 dpa. For irradiation at 300 K, an ion fluence of 0.40 Au₂⁺/nm₂(~1.1 dpa) is necessary to achieve an amorphous state at the damage peak. An analysis of the defects dechanneling factor suggests that the irradiation regions consist mostly of interstitial atoms or amorphous clusters. In situ thermal annealing experiments were performed to study damage recovery processes up to a maximum temperature of 870 K. The thermal recovery processes occur over a broad temperature range, and the disorder created by low ion fluences, 0.10-0.27 Au₂⁺/nm₂, is almost completely recovered after annealing at 870 K.
2001. "Hydrogen-Damage Interactions in Yttrium-Stabilized Zirconia." Journal of Nuclear Materials 289(1-2):128-135. Abstract Hydrogen diffusion and accumulation in oxidized Zr and ZrO2/Zr interfaces lead to hydrogen induced cracking in Zr-based alloys that are extensively used in nuclear reactors. In this study, the interaction of hydrogen with irradiation damage in (001) single crystals of yttrium-stabilized zirconia, Y-ZrO2, has been investigated as a function of damage accumulation and annealing temperature. Samples were irradiated with 40 keV hydrogen ions at a temperature of 1220 K to ion fluences of 5x10 16 and 1x10 17 ions/cm2, and isochronal annealing experiments were performed in the temperature range from 300 to 770 K in 100 K steps. Damage accumulation and hydrogen profile measurements indicate unusual damage recovery behavior, pinning of hydrogen by damage, and surface deformation due to hydrogen blisters and bubbles.
2001. "The Structure of Na2O-Al2O3-SiO2 Glass: Impact on Sodium Ion Exchange in H2O and D2O." Journal of Non-crystalline Solids 296:10-26. Abstract The kinetics of matrix dissolution and alkali-exchange for a series of sodium aluminosilicate glass compositions was determined at constant temperature and solution pH(D) under conditions of silica-saturation. Steady state release rate for sodium was 10 to 50 times faster than the rate of matrix dissolution, demonstrating that alkali exchange is an important long-term reaction mechanism that must be considered when modeling systems near saturation with respect to dissolved glass components. Sodium release rates were 30% slower in D2O compared to rates in H2O; but matrix dissolution rates were unaffected. These results are consistent with rupture of the O-H bond as the rate-limiting reaction in Na+-H+ exchange whereas matrix dissolution is controlled by OH- or H2O catalyzed hydrolysis of Si-O-Si and Si-O-Al bonds. Changes in Na exchange rate with increasing Al2O3 content could not be reconciled with changes in the number of non-bridging oxygen (NBO) sites in the glass alone. A simple model was used to estimate a structural energy barrier for alkali ion exchange using Na-O bond length and coordination as measured by Na K-edge XANES spectroscopy, and binding energy shifts for Si-O-Na sites measured by XPS. The energy barrier was calculated to increase from 34 kJ/mol for Na2O?2SiO2 glass to 50 kJ/mol for a glass containing 15 mol% Al2O3, consistent with stronger bonding of Na on NBO sites and increasing mechanical stiffness of the glass network with increasing Al content. The calculated ion-exchange enthalpies were then used to calculate Na ion exchange rates as a function of glass composition. Agreement between the calculated and measured Na ion exchange rates was excellent.
2001. "Accumulation and Recovery of Disorder on Silicon and Carbon Sublattices in Ion-Irradiated 6H-SiC." Journal of Nuclear Materials 289(1-2):96-101. Abstract Irradiation experiments have been performed at 100, 170 and 300 K for 6H-SiC single crystals using Au₂⁺and He+ ions over a range of fluences. The evolution of disorder on the both Si and C sublattices has been simultaneously investigated using 0.94 MeV D⁺ Rutherford backscattering spectrometry in combination with 12C(d,p) nuclear reaction analysis in a <0001> axial channeling geometry. The results show that the dependence of disorder on dose is consistent with a combined direct-impact / defect-stimulated model. At low doses, a slightly higher rate of C disordering is observed, which is consistent with molecular dynamics simulations that suggest a smaller threshold displacement energy on the C sublattice. At higher doses, the rate of C disordering decreases more rapidly than the rate of Si disordering, which suggests a higher rate of dynamical recovery on the C sublattice under the irradiation conditions. Three distinct recovery stages are observed on both the Si and C sublattices in the Au₂⁺-irradiated 6H-SiC. However, complete recovery of irradiation-induced disorder does not occur during isochronal annealing at temperatures up to 970 K.
1998. "Epitaxial Growth of fcc Ti Films on Al(001) Surfaces." Physical Review. B, Condensed Matter 56(15):9841-9847. Abstract Studies the growth of thin Ti films on Al(001) surfaces with high-energy ion scattering (HEIS), x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction (XPD). Observes that although there is a general similarity of fcc Ti growth on both Al(001) and Al(110), the submonolayer growth regime does show differences for the two surfaces.