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. "Crystal and Electronic Structure of Lithiated Nanosized RutileTiO2 by Electron Diffraction and Electron Energy-loss Spectroscopy." Applied Physics Letters 94(23):Art. No.: 233116. doi:10.1063/1.3152783 Abstract The electronic structure of the nanosized rutile TiO2 before and after mechanical lithiation were studied using TEM and EELS. EELS reveals the Li K-edge at the energy-loss position of ~ 61 eV. After lithiation, the separation of the t2g-eg crystal-field splitting on both Ti L2,3-edge and O K-edge decreases, the O K-edge shifts towards a higher energy-loss position and the separation between the pre-edge peak and main peak on the O K-edge decreases. These results suggest that the lithiation of rutile TiO2 was accompanied by the reduction of Ti ion and a charge transfer from Li to Ti.
2009. "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. "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. "Defect Interactions and Ionic Transport in Scandia Stabilized Zirconia." Physical Chemistry Chemical Physics. PCCP 11(26):5506-5511. Abstract Atomistic simulation has been used to study ionic transport in scandia-stabilized zirconia, as well as scandia and yttria-co-doped zirconia, as a function of temperature and composition. The oxygen diffusion coefficient shows a peak at a composition of 6 mole % Sc2O3. Oxygen vacancies prefer to be second nearest neighbours to yttrium ions, but have little preference between first and second neighbour positions with respect to scandium ions. The Sc-O bond length is about 2.17 Å compared to 2.28 Å for the Y-O bond. Oxygen migration between cation tetrahedra is impeded less effectively by Sc-Sc edges than by Y-Y edges. A neutral cluster of two scandium ions with an oxygen vacancy in the common first neighbour position has a binding energy of -0.56 eV. The formation of such clusters may contribute to conductivity degradation of stabilized zirconia at elevated temperature.
2009. "New Approaches for Characterizing Sensor and Other Modern Complex Materials." ECS Transactions 19(6):137-148. doi:10.1149/1.3118546 Abstract Advances in understanding of sensor and other modern complex materials are often enabled by new research tools. This paper highlights three capability development themes used to identify new research tools to be provided to users of the U. S. Department of Energy’s Environmental Molecular Sciences Laboratory. These capability development directions address the importance of dynamic measurements in realistic environments, the need for increased resolution in three dimensional analyses as well as the importance of linking theory and experiment. Capability development involves expanding the range of operation for a number of important techniques, developing and applying new capabilities, and advancing methods of data processing. Examples of current developments are provided including those related to magnetic resonance, x-ray diffraction, application of a focused beam capability to fuel cell aging, and near real time analysis of XPS spectra.
2008. "Growth and structure of epitaxial Ce0.8Sm0.2O1.9 by oxygen-plasma-assisted molecular beam epitaxy." Journal of Crystal Growth 310(2008):2450-2456. Abstract The epitaxial growth of Ce0.8Sm0.2O1.9 films on sapphire (0001) substrate by oxygen-plasma-assisted MBE has been characterized using RHEED, XPS, XRD, AFM, HRTEM and RBS in order to determine their structure and compositions. The composition of the films was determined to be Ce: Sm: O of 0.8:0.2:1.9 by RBS. The film/substrate epitaxial relationship can be written as CeO2 (111)// -Al2O3 (0001) and CeO2 [110]// -Al2O3 . The Ce has only 4+ oxidation state in the films and Sm is fully oxidized in the films with formal oxidation of 3+. CeO2 (111) face is preferred orientation and the thin films are cubic phases.
2008. "Conductivity of Oriented Samaria-Doped Ceria Thin Films Grown by Oxygen-plasma-assisted Molecular Beam Epitaxy." Electrochemical and Solid-State Letters 11(5):B76-B78. doi:10.1149/1.2890122 Abstract We have used oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) to grow highly oriented Ce1-xSmxO2-δ films on single crystal c-Al2O3. The samarium concentration, x, was varied in the range 1-33 atom%. It was observed that dominant (111) orientation in Ce1-xSmxO2-δ films can be maintained up to about 10 samarium atom% concentration. Films higher than 10 atom% Sm concentration started to show polycrystalline features. The highest conductivity of 0.04 S.cm-1, at 600 0C, was observed for films with ~ 5 atom% Sm concentration. A loss of orientation, triggering an enhanced grain boundary scattering, appears to be responsible for the decrease in conductivity at higher dopant concentrations.
2008. "Self-Assembly of Cerium Oxide Nanostructures in Ice Molds." Small 4(8):1210-1216. doi:10.1002/smll.200800219 Abstract The formation of nanorods, driven by the physico-chemical phenomena during the freezing of ceria nanoparticle suspension is reported. During freezing a dilute solution of CeO2 nanocrystals, some nuclei remain in solution while others are trapped inside the voids formed within the growing ice front. Over time the particles trapped within the constrained geometries combined by an oriented attachment process to form ceria nanorods. The experimental observations are further supported through Molecular Dynamics (MD) simulations. These observations suggest a new possible strategy for the templated formation of nanostructures through self assembly by exploiting natural phenomena such as freezing of water. "(A portion of) The research described in this paper (poster or presentation) was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory."
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. "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. "Synthesis and Characterization of Compositionally Graded Si1-xGex Layers on Si substrate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 261(1-2):723-726. Abstract Thin film of silicon germanium (Si1-xGex) with tailored composition was grown on Si (100) substrate at 650oC in an ultrahigh vacuum molecular beam epitaxy system. The nominal x-value is ranged from 0 to 0.14. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDX), and atomic force microscopy (AFM). RBS/Channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. Both RBS and EDX analyses indicate the compositional graded incorporation of Ge in the film with x ranging from 0 to 0.14. The film shows island growth with each island centering around an interface dislocation.
2007. "Epitaxial Growth and Microstructure of Cu2O Nanoparticle/thin Films on SrTiO3(100)." Nanotechnology 18:Art. No. 115601. doi:10.1088/0957-4484/18/11/115601 Abstract Cuprous oxide (Cu2O) was grown on SrTiO3 (STO)(100) by oxygen plasma assisted molecular beam epitaxy. Microstructure of the grown layer and Cu valence state were analyzed using x-ray diffraction (XRD), x-ray photo-electron spectroscopy (XPS), atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) as well as electron diffractions. The grown layer was dominated by Cu2O phase, possessing an epitaxial orientation with the substrate such that: Cu2O[001]//STO[001] and Cu2O(100)//STO(100). Cu2O film morphologically shows dependence on the growth rate. Typically, a fast growth will lead to the formation of a thin film with a relatively smooth surface. A slow growth will lead to the development of nanoparticles, featuring the formation of Cu2O pyramid. The pyramids are invariantly defined by the Cu2O {111} planes. Given the fact that the {111} planes correspond to the lowest surface energy of Cu2O, a slow growth will lend the system enough time to allow it to adopt the pyramid configuration by which the overall energy of the system was minimized.
2007. "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. "Adsorption of iso-/n-butane on an Anatase Thin Film: A Molecular Beam Scattering and TDS Study." Catalysis Letters 116(1-2):9-14. doi:10.1007/s10562-007-9121-x Abstract Binding energies and adsorption probabilities have been determined for n/iso-butane adsorption on an anatase thin film grown on SrTiO3(001) by means of thermal desorption spectroscopy (TDS) and molecular beam scattering. The sample has been characterized by x-ray diffraction (XRD) and Auger electrons spectroscopy (AES).
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. "Reproducible Tip Fabrication and Cleaning for UHV STM ." Ultramicroscopy 108(9):873-877. doi:10.1016/j.ultramic.2008.02.010 Abstract Several technical modifications related to the fabrication and ultra-high vacuum (UHV) treatment of the tips have been implemented to improve a reliability of the tip preparation for high-resolution scanning tunneling microscopy. The widely used drop-off technique for the tip electrochemical etching has been further refined to enable a reproducible fabrication of the tungsten tips with a radius 3 nm. Simple and flexible setup for the tip UHV annealing has been developed and employed. The Ar ion sputtering with subsequent annealing has been adopted for a final tip treatment. The proper tip preparation has been demonstrated by imaging an atomic structure of the rutile TiO2(110) surface.
2006. "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. "Morphological Evolution of Ba(NO3)2 Supported on -Al2O3(0001): An In-Situ TEM Study." Journal of Physical Chemistry B 110(24):11878-11883. doi:10.1021/jp060235i Abstract One of the key questions for the BaO-based NOx catalyst system is the morphological evolution of Ba(NO3)2 to BaO upon heating for releasing of NOx or vice versa from BaO to Ba(NO3)2 upon uptaking of NOx. However, associated with the small crystallite size of high-surface area Al2O3, it can be difficult to extract structural and morphological features of Ba(NO3)2 supported on -Al2O3 by any direct imaging method including transmission electron microscopy. In this work, by choosing a model system of Ba(NO3)2 particles supported on single crystal -Al2O3, we have investigated the structural and morphological features of Ba(NO3)2 as well as the formation of BaO from Ba(NO3)2 during the release of NOx using ex-situ and in-situ TEM imaging, electron diffraction, energy dispersive spectroscopy (EDS), and Wulff shape construction. We find that Ba(NO3)2 supported on -Al2O3 possesses a platelet morphology, with the interface and facets being invariably the 8 {111} planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)2 and -Al2O3, indicating that the interfacial energy is lower than the Ba(NO3)2 surface free energy. In fact, Wulff shape constructions indicate that the interfacial energy is ~1/4 of the {111} surface free energy of Ba(NO3)2. The orientation relationship between Ba(NO3)2 and the -Al2O3 is: -Al2O3[0001]//Ba(NO3)2[111] and -Al2O3(1-2 10)//Ba(NO3)2(110).
2006. "Distribution of Oxygen Vacancies and Gadolinium Dopants in ZrO2-CeO2 Multi-Layer Films Grown on α-Al2O3." Solid State Ionics 177(15-16):1299-1306. doi:10.1016/j.ssi.2006.05.036 Abstract Gd-doped ZrO2 and CeO2 multi-layer films were deposited on α-Al2O3 (0001) using oxygen plasma assisted molecular beam epitaxy. Oxygen vacancies and Gd dopant distributions were investigated in these multi-layer films using x-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), annular dark-filed imaging in scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping and x-ray photoelectron spectroscopy (XPS) depth profiling. EDS and XPS reveal that Gd concentration in the ZrO2 layer is lower than that in the CeO2 layer. As a result, higher oxygen vacancy concentration exists in CeO2 layers compared to that in ZrO2 layers. In addition, Gd is found to segregate only at the interfaces formed during the deposition of CeO2 layers on ZrO2 layers. On the other hand, the interfaces formed during the deposition of ZrO2 layers on CeO2 layers did not show any Gd segregation. The Gd segregation behavior at the every other interface is believed to be associated with the low solubility of Gd in ZrO2.
2006. "Atomic level imaging of Au nanocluster dispersed in TiO2 and SrTiO3 ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):380-382. doi:10.1016/j.nimb.2005.08.144 Abstract Au nanoclusters dispersed in single crystal TiO2 and SrTiO3 have been prepared by ion implantation at 300 and 975 K and subsequent annealing at 1275 K for 10 hours. High resolution transmission electron microscopy and high-angle-annular-dark-field (HAADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) have been used to characterize the microstructure of the dispersed gold nanoclusters. The results indicate that Au atoms substitute for cations in these systems. Cavities of up to several tens of nanometers are observed in TiO2 and SrTiO3. The nanometer-sized cavities and Au clusters are faceted along the same lattice plane of the matrix, indicating that the interfacial energy (defined by the Au cluster and the matrix) and the surface energy of the matrix (defined by cavity and the matrix ) follow a similar trend of change with respect to different lattice planes of the matrix.
2006. "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. "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. "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. "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. "Partial encapsulation of Pd particles by reduced ceria-zirconia." Applied Physics Letters 87(20):Art. No. 201915. doi:10.1063/1.2132067 Abstract The interaction between metal particles and their oxide support can be strong so as to affect the reactivity of a catalyst system by, for example, encapsulation of the particles by the oxide. Direct observation of metal-oxide interfaces with atomic resolution is a challenge and can only be achieved by cross sectional high-resolution transmission electron microscopy (HRTEM). With this approach, we found partial encapsulation of Pd particles by reduced ceria-zirconia in a model, single-crystal thin film auto catalyst, indicating a strong interaction between Pd and the oxide. Besides obtaining HRTEM images, the valence of cerium was determined by electron energy loss spectroscopy (EELS). The effect of reduction and oxidation conditions on this interaction provides a qualitative explanation for a previously observed reversible reactivation of oxygen storage in model powder auto catalysts. The technique of cross sectional HRTEM can be applied to the study of other metal-particle-on-oxide systems.
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. "Hydrogen Bubbles and Formation of Nanoporous Silicon during Electrochemical Etching." Surface and Interface Analysis 37(6):555-561. Abstract Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etch nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of a heavily n-doped (R = 0.05-0.5 Ω -cm) <100> silicon at a current density of 10 mA/cm² in buffer oxide etch (BOE) electrolyte solution defect craters, containing textured nanopores, were observed to occur in ring shaped patterns of rings. The defect craters apparently originate at the hydrogen-BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current carrying Si-OH species are most likely responsible for the widening in the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the low density of nanopores formed at ring edges are isolated from each other. Overall features observed in photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) intensity strongly emphasize the role of surface oxide that influences these properties.
2005. "Ceria Incorporation into YSZ Columnar Nanostructures." Electrochemical and Solid-State Letters 8(10):A525-A527. Abstract We report the growth of porous yttria-stabilized zirconia (YSZ) columnar nanostructures by glancing angle deposition (GLAD) technique. CeO₂ sol solution was incorporated into YSZ creating high interface density columnar nanostructures. Initial experiments suggest higher conductivity in CeO₂/YSZ columnar nanostructures than polycrystalline CeO₂ and lower conductivity than single crystal YSZ in the intermediate temperature range of 600-825 K. GLAD/sol-gel process combination to create high density columnar nanostructures is discussed in the context of solid oxide fuel cells operating at intermediate temperatures.
2005. "Development of High-Temperature Ferromagnetism in SnO₂ and Paramagnetism in SnO by Fe Doping." Physical Review. B, Condensed Matter and Materials Physics 72(5):054402 (14 p.). Abstract We report the development of room-temperature ferromagnetism in chemically synthesized powder samples of Sn₁-xFexO₂ (0.005 ≤ x ≤ 0.10) and paramagnetic behavior in an identical set of Sn₁-xFexO. The ferromagnetic Sn₀․₉₉Fe₀․₀₁O₂ showed a Curie temperature Tc = 850 K, which is among the highest reported for dilute magnetic semiconductors. No evidence of dopant segregation was detected in Sn₁-xFexO₂ or Sn1-xFexO, suggesting that the emerging magnetic interactions in these systems are strongly related to the properties of the host systems SnO and SnO₂.
2005. "Development of high-temperature ferromagnetism in SnO2 and paramagnetism in SnO by Fe doping." Virtual Journal of Nanoscale Science & Technology 12(7):, Abstract We report the development of room-temperature ferromagnetism in chemically synthesized powder samples of Sn1−xFexO2 *0.005*x*0.05* and paramagnetic behavior in an identically synthesized set of Sn1−xFexO. The ferromagnetic Sn0.99Fe0.01O2 showed a Curie temperature TC=850 K, which is among the highest reported for transition-metal-doped semiconductor oxides. With increasing Fe doping, the lattice parameters of SnO2 decreased and the saturation magnetization increased, suggesting a strong structure-magnetic property relationship. When the Sn0.95Fe0.05O2 was prepared at different temperatures between 200 and 900 °C, systematic changes in the magnetic properties were observed. Combined Mössbauer spectroscopy and magnetometry measurements showed a ferromagnetic behavior in Sn0.95Fe0.05O2 samples prepared at and above 350 °C, but the ferromagnetic component decreased gradually as preparation temperature approached 600 °C. All Sn0.95Fe0.05O2 samples prepared above 600 °C were paramagnetic. X-ray photoelectron spectroscopy, magnetometry, and particle induced x-ray emission studies showed that the Fe dopants diffuse towards the surface of the particles in samples prepared at higher temperatures, gradually destroying the ferromagnetism. Mössbauer studies showed that the magnetically ordered Fe3+ spins observed in the Sn0.95Fe0.05O2 sample prepared at 350 °C is only *24% of the uniformly incorporated Fe3+. No evidence of any iron oxide impurity phases were detected in Sn1−xFexO2 or Sn1−xFexO, suggesting that the emerging magnetic interactions in these systems are most likely related to the properties of the host systems SnO2 and SnO, and their oxygen stoichiometry.
2005. "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. "Near-edge X-ray Absorption Fine Structure Study of Disordering in Gd₂(Ti1-yZry)₂O₇ Pyrochlores." Journal of Physical Chemistry B 109(4):1337-1339. Abstract Disorder in Gd₂(Ti1-yZry)₂O₇ pyrochlores, for y=0.0-1.0, is investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure spectroscopy. Ti⁴⁺ ions are found to occupy octahedral sites in Gd₂Ti₂O₇ with a tetragonal distortion induced by vacant oxygen sites. As Zr substitutes for Ti, the tetragonal distortion decreases, and Zr coordination increases from 6 to 8. The migration of oxygen ions from 48f or 8b sites to vacant 8a sites compensate for the increased Zr coordination, thereby reducing the number of vacant 8a sites, which further reduces the tetragonal distortion and introduces more disorder around Ti. This is evidence for simultaneous cation disorder with anion migration.
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. "Formation of Epitaxial Oxide Nanodots on Oxide Substrate: Cu₂O on SrTiO₃(100)." Surface Science 589(1-3):120-128. Abstract X-ray photoelectron spectroscopy analysis during the oxygen plasma assisted molecular beam epitaxy, combined with atomic force microscopy, scanning Auger microscopy, and theoretical simulation studies have been used to evaluate the mechanism of single-phase Cu₂O nanodot formation on the SrTiO₃(100) surface. Formation of pure crystalline Cu₂O nanodots occurs rather in a narrow growth parameter window, outside which a coexistence of the multiple phases has been observed. Cuprous oxide nanodots on the SrTiO₃(100) substrate follow a growth mechanism which differs significantly from the growth modes observed for the majority of semiconductor quantum dots. Growth starts without wetting layer formation with appearance of well-ordered truncated square-based nanodots at submonolayer coverages. At the initial stages of growth, the nanodot size is only weakly changes with coverage and exponentially scales with temperature. After reaching a critical, temperature dependent dot density (~ 10¹³ cm-² for 760 K growth temperature), growth of mid-sized nanoclusters starts through coalescence, which is eventually followed by large dome-shaped cluster formation at higher coverages. The coexistence of the different types of the clusters at high coverages results in a multi-modal distribution of sizes and shapes.
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. "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. "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. "Lateral Ordering of Microfabricated SiO₂ Nanotips." Electrochemical and Solid-State Letters 7(1):C7-C9. Abstract Micro -fabricated SiO₂ nano - tips are potentially useful as scanning tips in near field optical microscopy and sensor related functions. We report a process in which the rounding nature of isotropic etching is effectively used to micro -fabricate laterally ordered SiO₂ nano -tips. Combination of excessive isotropic wet etching of thermally grown SiO₂ with anisotropic etching of n type silicon along <100> planes leads to the formation of nano -tips with sharpness ~ 15 nm. Uniform periodic array of nano -tips form due to coalescence of excessively etched SiO₂ resulting in nano -tips length the separation between the original photolithographic features. Finally, the overall process of nano -tip formation is discussed by considering the roles of rapid isotropic etching of SiO₂ in buffered oxide etch solution, anisotropic etching of Si <100> in KOH solution, and slow SiO₂ etching in KOH solution.
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. "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. "Initial Stages of Oxide Nanodot Heteroepitaxial Growth: Cu2O on SrTiO3(100)." Applied Physics Letters 85(19):4481-4483. Abstract The growth mechanism in a heteroepitaxy of oxide nanodots is investigated by combination of the x-ray photoelectron spectroscopy (XPS), atomic force microscopy and theoretical modeling. In contrast to the majority of semiconductor systems, in the studied metal oxide system of Cu2O – SrTiO3(100) the growth process starts without wetting layer formation with appearance of small ( ~ 10 nm) square-based planar Cu2O nanodots. Continued deposition leads mainly to increase of the nanodot density, practically, without change of their size. Only after reaching some critical density (~ 1013 cm-2 for 760 K growth temperature), growth of scattered, significantly larger islands starts through the coalescence of small nanodots. XPS analysis suggests that the interface between small nanodots and substrate is abrupt with only weak Cu – O(SrTiO3) interaction.
2004. "Formation of Single-Phase Oxide Nanoclusters: Cu₂O on SrTiO₃(100)." Journal of Applied Physics 94:7926. Abstract Selective formation of the single phase nano-clusters of Cu₂O on SrTiO₃(100) substrates in the size range of 10-50 nm is found to occur only in a very narrow oxygen plasma assisted molecular beam epitaxy growth parameter window, in comparison with the bulk phase diagram (for oxygen pressure vs. temperature). X-ray photoelectron spectroscopy, Auger electron spectroscopy and x-rays diffraction analysis of nano-clusters, has detected the distinctive parameter regions, where multiple phase-like forms coexist (CuO/Cu₂O and Cu₂O/Cu), in agreement with theoretical prediction for small systems, and as opposite to the sharp phase boundaries for the bulk. Observed changes in the nano-cluster composition are found to correlate with differences in cluster morphologies.
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. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced CeO₂(111) Surfaces." Surface Science Spectra 11(1-4):73-81. Abstract We have studied the electronic structure of oxidized and reduced CeO₂ (111) surfaces using X-ray photoelectron spectroscopy (XPS). The 50 nm thick Co₂(111) film was grown on a YSZ(111) substrate using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (X-ray diffraction), HRTEM (high resolution transmission electron microscopy) and RBS (Rutherford backscattering spectroscopy). The lattice mismatch between CeO₂(111) and YSZ(111) is less than 5% and yields a flat surface that is comprised of an equivalent number of Ce⁴⁺ and O₂₋ ions. Oxidation with O₂ at 773 K under UHV conditions was sufficient to fully oxidize the CeO₂(111). Surface reduction was carried out by annealing in UHV at 973 K.
2004. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced Ce₀․₈Zr₀․₂O₂(111)." Surface Science Spectra 11(1-4):82-90. doi:10.1116/11.20050202 Abstract We have studied the electronic structure of oxidized and reduced Ce₀․₈Zr₀․₂O₂(111) using x-ray photoelectron spectroscopy (XPS). The 50 nm thick Ce₀․₈Zr₀․₂O₂(111) film was grown on a YSZ(111) substrate using oxygen assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (x-ray diffraction), HRTEM (high energy resolution transmission electron spectroscopy) and RBS (Rutherford backscattering spectroscopy). Surfaces of the Ce₀․₈Zr₀․₂O₂(111) film used in this study is found to be unreconstructed and exhibits the structure of bulk CeO₂(111) where Zr atoms occupy the lattice sites of Ce in the fluorite structure of ceria. The extent of surface reduction as a result of vacuum annealing has been reported here in addition to the electronic structure of defect-free Ce₀․₈Zr₀․₂O₂(111) surface.
2003. "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 Structure of an Epitaxial Cubic Ceria Film on Cubic Zirconia." Journal of the American Ceramic Society 86(2):363-365. Abstract A cubic CeO₂ (001) film of thickness equal to ~58 nm was epitaxially grown on Y₂O₃-stablized cubic ZrO₂ by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface was characterized using high resolution transmission electron microscopy (HRTEM). The interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the [100] direction, the dislocation spacing is 3.3 ? 0.5 nm, which is slightly shorter than the expected value of 4.9 nm calculated from the differences in lattice constants given in the literature, but is fairly consistent with the 3.9 nm lattice mismatch measured by electron diffraction. Thus, the results presented here indicate that the lattice mismatch between the film and the substrate is accommodated mainly by interface misfit dislocations above some critical thickness.
2003. "Interface Characteristics of Iso-Structural Thin Film and Substrate Pairs." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):1-9. Abstract Cubic-CeO₂ and -Fe₂O₃ thin films have been epitaxially grown on yttria-stabilized ZrO₂ and -Al₂O₃ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by Rutherford backscattering spectrometry (RBS), high resolution transmission electron microscopy (HRTEM), and x-ray diffraction (XRD). RBS channeling spectra for both CeO₂/ZrO₂ and Fe₂O₃/Al₂O₃ show interface disorder-related scattering peaks. It is believed that the observed interface disorder-related scattering peaks on RBS spectra are attributed to the interface misfit dislocations. Cross sectional HRTEM reveals that interfaces of both systems are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. The experimentally observed dislocation spacings are approximately consistent with those calculated from the lattice mismatch, implying that the lattice mismatch is accommodated mainly by interface misfit dislocations above the critical thickness.
2003. "Editorial." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):vii. Abstract Ion Beam Processing and Modification of Glasses and Ceramics was held as one of the symposia in Advanced Materials Synthesis and Processing Technology session, 104th American Ceramic Society Meeting in St. Louis from April 28 to May 1, 2002. Three divisions of American Ceramic Society (Basic Sciences, Glass and Optical Materials, and Nuclear and Environmental Technology Divisions) and Thermionics NW, Inc. jointly sponsored the symposium. This one and a half day symposium brought together several scientists from many di.erent parts of the globe to exchange ideas and share common activities in this .eld. The application of ion beams to the processing and modi.cation as well as characterization of materials has led to the evolvution of highly versatile characterization and processing techniques for a wide-ranging utilization in many .elds. The symposium featured the following focused sessions: (1) Ion Beam Analysis of Ceramics and Glasses, (2) Ion Beam Processing of Glasses and (3) Ion Beam Processing of Ceramics. Each of this focused sessions had two invited speakers and 4–6 contributed speakers. In addition, a poster session was organized to accommodate the papers, which could not be presented due to the time limitation of the symposium. About half of the presenters contributed manuscripts to the Proceedings. This special issue of Nuclear Instruments and Methods in Physics Research B contains the proceedings of this symposium. It is our pleasure to acknowledge the contributing authors for the preparation of the manuscripts and for their prompt response to our urgent messages. We also thank all of our reviewers for their timely conscientious and critical reviewing of the manuscripts. Timely responses and support from the authors and reviewers have enabled the publication of the Proceedings as scheduled. We are indebted to our sponsors, the American Ceramics Society and Thermionics NW, Inc., without whose support the symposium could not have been organized.
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. "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.
2003. "Effect of Ion Irradiation in Cadmium Niobate Pyrochlores." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):85-91. Abstract Irradiation experiments have been performed for cadmium niobate pyrochlore (Cd₂Nb2O₇) single crystals at both 150 and 300 K using 1.0 MeV Au₂⁺ ions over fluences ranging from 0.01 to 0.10 ions/nm₂. In-situ 3.0 MeV He⁺ Rutherford backscattering spectrometry along the <100>-axial channeling direction (RBS/C) has been applied to study the damage states ranging from small defect concentrations to a fully amorphous state. Results show that the crystal can be readily amorphized under the irradiation conditions. Room-temperature recovery of the defects produced at 150 K has been observed, while the defects produced at 300 K are thermally stable at room temperature. Results also indicate that the RBS/C analysis used in this study induced negligible damage in the near-surface regime. In addition, irradiation at and below room temperature using He⁺ and C3⁺ ions leads to surface exfoliation at the corresponding damage peaks.
2003. "Redox Properties of Water on the Oxidized and Reduced Surfaces of CeO₂ (111)." Surface Science 526(1-2):1-18. Abstract We present x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) results probing the surface chemistry of water on the oxidized and reduced surfaces of a 500 Å epitaxial CeO₂ (111) film grown on yttria-stabilized ZrO₂ (111). Oxidation with O₂ at 773 K under UHV conditions was sufficient to generate XPS spectra reflective of fully oxidized CeO₂ (111). Surface reduction was carried out by annealing in UHV between 773 and 973 K, and the level of reduction was quantified using changes in the Ce₃d₃/₂ 4f0 photoemission peak at 917 eV which results primarily from Ce₄⁺ sites. As expected, the level of surface reduction (generation of Ce₃⁺ sites) increased with increasing temperature. These Ce₃⁺ sites were primarily in the first layer based on the fact that exposure of the film to O₂ at RT resulted in nearly complete conversion of Ce₃⁺ to Ce₄⁺. Annealing at 773 K led to a surface in which approximately 40% of the surface Ce₄⁺sites were reduced to Ce₃⁺, whereas annealing at higher temperatures led to more substantial reduction of the first layer along with some subsurface reduction that was not reoxidized by RT exposure to O₂.
2003. "Atomic-Level Simulations of Misfit Dislocation at the Interface of Fe2O3/Al2O3 System." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):63-71. Abstract When -Fe2O3 thin films are deposited on -Al2O3 (0001) substrates using oxygen plasma assisted molecular beam epitaxy (OPA-MBE), a periodic distribution of basal dislocations occurs due to lattice mismatch along the interfaces. High-resolution transmission electron microscopy (HRTEM) shows, when observed from zone axis, that these dislocations lie at the interface about 7.0 nm apart. Molecular dynamics simulations (MD) were performed in order to understand the formation of misfit dislocations and the interface structural features in Fe2O3/Al2O3 system. It is found that the misfit dislocations are mainly formed in Al2O3 substrates with Burger's vector of 1/3< >, and terminated at the interfaces, in consistent with experimental observations. These dislocations can dissociate into two partial dislocations with Burger's vectors of 1/3< > and 1/3< > by forming stacking faults on (0001) planes. The core structures of the misfit dislocations in semicoherent interfaces are analyzed in detail, and the misfit dislocations narrow cores in the plane of the interfaces.
2002. "The Characteristics of Interface Misfit Dislocations for Epitaxial alpha-Fe₂O₃ on alpha-Al₂O₃(0001)." Thin Solid Films 414:31-38. Abstract Alpha-Fe₂O₃(0001) films of thickness equal to ~7 nm and ~70 nm were epitaxially grown on alpha-Al₂O₃(0001) by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interfaces were characterized using high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and x-ray diffraction (XRD). The interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the [2110] direction, the dislocation spacing is 7.0 +- 1.1 nm for the 70 nm thick speciman, and 7.2 +- 0.1 nm for the 7 nm thick speciman. When imaged from the [0110] direction, the disslocation spacing is 4.5 +- 0.1 nm for the 7 nm thick speciman. The experimentally observed dislocation spacings are appproximately consistent with those calculated from the lattice mismatch between alpha-Al₂O₃ and alpha-Fe₂O₃, implying that the lattice mismatch is accomodated mainly be interface misfit dislocations above the critical thickness, which is less than 7 nm. This conclusion is also corroborated by the measured reidual strainof ~0.5% determined from x-ray diffraction for the 70 nm film . EELS analysis reveals that the Fe L₂,3-edge shows no measurable chemical shift relative to the L₂,3-edge of structural Fe₃⁺, indicating complete oxidation of Fe in the as-grown film.
2002. "HRTEM Characterization of Interface Between Iso-Structural Thin Solid Film and Substrate." Microscopy & Microanalysis 8(Suppl S02):1160-1161. doi:10.1017/S1431927602107975 Abstract Alpha-Fe₂O₃ and c-CeO₂ thin films have been epitaxially grown on alpha-Al₂O₃ and yttria- stabilized c-ZrO₂ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), Rutherford backscattering spectrometry (RBS), and x-ray diffraction (XRD). For the two systems studied, the interfaces are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. These results will be presented along with the results from molecular dynamics (MD) simulations of these interfaces.
2002. "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, Dynamic Annealing and Thermal Recovery of Ion-Beam-Induced Disorder in Silicon Carbide." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 175-177:26-30. Abstract Ion-beam-induced disordering in single crystals of 6H-SiC has been investigated for a wide range of ion species (from H⁺ to Au2⁺) using in situ ion-channeling methods. Silicon carbide is readily amorphized below room temperature with all ions. The rate of ion-beam-induced disordering decreases with decreasing ion mass and with increasing temperature. Analysis of limited data suggests that the activation energy for dynamic recovery during irradiation below 300 K is on the order of 0.1 eV. Thermal annealing indicates similar three-stage recovery on both the Si and C sublattices, which suggests similar recovery processes and activation energies. The activation energies for thermal recovery on the Si sublattice are estimated to be 0.3 ? 0.15 eV (Stage I), 1.3 ? 0.25 eV (Stage II), and 1.5 ? 0.3 eV (Stage III).
2001. "Cleaving Oxide Films Using Hydrogen Implantation." Materials Letters 49(6):313-317. Abstract Precise cleaving of oxide films with known thickness using hydrogen implantation and subsequent annealing was investigated using strontium titanate (SrTiO₃) as a model material. Rutherford backscattering in channeling geometry (RBS/C), nuclear reaction analysis (NRA), and scanning electron microscopy (SEM) have been used to characterize this process.
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. "Ion Implantation and Thermal Annealing in Silicon Carbide and Gallium Nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 178:204-208. Abstract Ion-beam-induced disordering in single crystals of 6H-SiC and single-crystal films of GaN has been investigated using ion-channeling methods. Amorphization in GaN requires a dose that is about 30 and 100 times higher than in SiC at 180 and 300 K, respectively. Dynamic defect-recovery processes in both materials increase with increasing irradiation temperature. Amorphization in SiC is consistent with a combined direct-impact and defect-stimulated process. Three recovery stages are observed on both the Si and C sublattices under isochronal annealing in Au₂⁺-irradiated 6H-SiC. In GaN, an intermediate saturation state is observed for disordering at the damage peak, which suggests enhanced defect annihilation processes. Implanted Au diffuses towards the surface during implantation at 300 K and undergoes further diffusion into the amorphous surface layer during post-implantation annealing at 870 K.
2001. "Damage Accumulation and Recovery in Gold-Ion-Irradiated Barium Titanate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 175:610-614. Abstract Single-crystal barium titanate (BaTiO₃) wafers were irradiated 60? off the surface normal at 170 and 300 K using 1.0 MeV Au₂⁺ ions over a fluence range from 0.03 to 0.19 ions/nm₂. Disorder on both the Ba and Ti sublattices has been studied in situ using Rutherford backscattering spectrometry along the <110> axial direction. At these irradiation temperatures, the temperature dependence of disordering is small. The dose for amorphization under these conditions is on the order of 0.5 dpa, which is 50% of that required to amorphize SrTiO₃ under similar conditions. At low damage levels, recovery of disorder is observed at room temperature, suggesting at least one lower temperature recovery stage. For more highly damaged states, two distinct recovery stages have been identified between 420 and 570 K and between 720 and 870 K. The recovery stage between 420 and 570 K is associated with the critical temperature for full amorphization (~550 K) in BaTiO₃. The higher temperature recovery stage is most likely associated with epitaxial recrystallization.
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.
2001. "Epitaxial Growth and Properties of Ferromagnetic Co-doped TiO2 Anatase." Applied Physics Letters 79(21):3467-3469. Abstract We have used oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) to grow CoxTi1-xO2 anatase on SrTiO3(001) for x= ~0.01-0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or disordered, these CoxTi1-xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1-xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous PLD-grown material.
2001. "Heavy-Ion Irradiation Effects on Structures and Acid Dissolution of Pyrochlores." Journal of Nuclear Materials 288(2-3):208-216. Abstract The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi+ ions, for A₂Ti₂O₇ pyrochlores, in which A=Y, Sm, Gd and Lu, exhibits no significant effect of A-site ion mass or size. The room temperature dose for amorphization was found to be ~ 0.18 dpa in each case. After irradiation with 2 MeV Au₂⁺ ions glancing-incidence XRD revealed that each pyrochlore underwent an irradiation-induced structural transformation to fluorite in conjunction with amorphization. The effect of amorphization on the dissolution rates of fully dense pyrochlores, at 90oC and pH 2 (nitric acid) varied from a factor of 10-15 increase for Gd₂Ti₂O₇ to none for Y₂Ti₂O₇. Significant differences were observed in the A-site dissolution rates from the crystalline pyrochlores, indicating differences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy.
2001. "Heavy-Ion Irradiation Effects in Gd2(Ti2-xZrx)O7 Pyrochlores." Journal of Nuclear Materials 289(1-2):188-193. Abstract Gd2(Ti2-xZrx)O7 samples with 0 = x = 1.5 were single-phase and pyrochlore structured after sintering at 1600oC in air. The Gd2Zr2O7 (x=2) end member was predominantly fluorite-structured. Raman spectroscopy indicated that the level of short-range fluorite-like disorder in the unirradiated Gd2(Ti2-xZrx)O7 samples increased significantly as Zr was substituted for Ti, despite the retention of a long-range pyrochlore structure for samples with 0 = x = 1.5. Glancing-incidence X-ray diffraction indicated that pyrochlores with an ionic radii ratio rA/rB = 1.52 (x=1.5) were transformed into a radiation resistant fluorite-structure after irradiation at room temperature with 2 MeV Au2+ to a fluence of 5 ions/nm2. As the ionic radii ratio of the pyrochlore increased beyond rA/rB > 1.52, the fluorite structure became increasingly unstable with respect to the amorphous state under identical irradiation conditions.
2000. "Interaction of Aqueous Chromium Ions with Iron Oxide Surfaces." Chapter 14 in Nuclear Site Remediation: First Accomplishments of the Environmental Management Science Program. ACS Symposium Series, vol. 778, ed. Eller, P. Gary; Heineman, William R., pp. 212-246. American Chemical Society, Washington DC. Abstract To gain a more fundamental understanding of abiotic processes controlling reduction reactions of aqueous chromate and dichromate ions (Cr(VI)aq) in subsurface environments, we carried out molecular-level experimental and modeling studies of the interaction of water and Cr(VI)aq with well-characterized single crystal samples of synthetic and natural hematite and magnetite. A reductionist approach was adopted in which simplified model systems of increasing complexity were studied. Photoemission spectroscopy (PES), photo-electron diffraction, and vacuum STM were used to characterize the composition, atomic structure, and morpho-logy of clean surfaces of ⍺-Fe₂O₃(0001) and Fe₃O₄(100) grown by molecular beam epitaxy on single crystal substrates of ⍺-Al₂O₃(0001) and MgO(100), respectively.
2000. "Defect Annealing Kinetics in Irradiated 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 166-167:410-414. Abstract Isochronal and isothermal annealing of ion-irradiation damage on the Si sublattice in 6H-SiC has been investigated experimentally by in-situ Rutherford Backscattering Spectrometry in channeling geometry (RBS/C). At low ion fluences corresponding to dilute concentrations of irradiation-induced defects, complete recovery of disorder on the Si sublattice can occur below room temperature. The implantation of helium impedes the defect recovery processes at low temperatures. Below room temperature, the thermal recovery of defects on the Si sublattice has an activation energy on the order of 0.25 eV. Recovery of disorder on the Si sublattice above 570 K has an activation energy on the order of 1.5 eV.
2000. "Ion Beam Analysis of Interface Reactions in Magnetite and Maghemite Thin Films." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:510-514. Abstract We have investigated inteface reactions between expitaxilly-grown magnetite (Fe₃O₄) and maghemite (gamma-Fe₂O₃) films with MgO substrates using Rutherford backscattering (RBS), channeling, and x-ray diffraction(XRD). Annealing these films in 2.0x10 -6 Torr of oxygen at temperatures up to 970 K enhances Mg outdiffusion into the films and increases the film thickness depending on temperature. The magnetite film thickness reach a limiting value at 870 K anneal while the maghemite film thickness did not maximize after annealing at 970 K. After the annealing at 970 K, both films produced a compound with composition close to magnesioferrite (MgFe₂O₄). XRD results reveal the formatioin of magnesioferrite (MgFe₂O₄) films after annealing both films at 970 in oxygen.
2000. "Investigation of Thermal Recovery Behavior in Hydrogen-Implanted SrTiO₃ Using High Energy Ion Beam Techniques." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:544-548. Abstract Perovskite materials, such as SrTiO₃, are of broad interest for applications ranging from electronic devices to the immobilization of high-level nuclear wastes. As a result, there is renewed interest in developing a fundamental understanding of irradiation and implatation effects on the properties and performance of SrTiO₃. In this study, low-energy (40 keV) H⁺ was implanted into single crystal SrTiO₃ at ~ 120 K over a range of ion fluences. Rutherford Backacattering Spectroscopy in channeling geometry (RBS/C) was used to investigate the ion-beam-induced disordering and recovery processed in the irradiated SrTiO₃. The resonant nuclear reaction, 1H(19F, alpha-gamma)16O, was used to profile the implanted H. For annealing temperatures up to ~570 K, isochronal annealing results indicate increasing disorder on the Sr, Ti, and O sublattices with temperature in the vicinity of the implanted hydrogen, possibly as a result of reaction of the implated hydrogen with the lattice. This increased disorder showed evidence for recovery at 570 K and decreased with further increases in annealing temperature up to 770 K. isochronal annealing at 870 K resulted in a uniform increase in the backscattering yield across the penetration depth of the implant hydrogen. Scanning electron microscopy (SEM) showed that blisters (or bubble-like platlets) of several microns diameter formed below the surface of the SrTiO₃ after the 870 K anneal; thus deforming the surface above the blister. The blisters most likely contain H₂. Such blistering is a well-known phenomenon in gas-implanted metals and was extensively investigated in the 1970's. This may be the first reported observation for blistering in a crystalline oxide. Secondary ion mass spectroscopy (SIMS), and Infrared spectroscopy (IR) measurements are planned to charaterize the nature of the chemical bonding in the implanted region. These results will be discussed in detail.
2000. "Effectiveness of High Energy Ion Beam Techniques for the Characterization of Mesoporous Low Dielectric-Constant Materials." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:476-481. Abstract There is growing interest interest in integration of low dielectric materials in microelectric devices. Highly porous silica films can potentially reduce power dissipation, cross talk, and interconnection delay in the deep submicron device regime. Recently, low k dielectric "mesoporous" silica films have been synthesized [1] using micellar surfactants to template porosity in spin-on sol-gel silica. Since then we have made significant progress in developing low k films, using both cationic and non-ionic surfactants, that could meet device performance requirements. During this development we have used high-energy ion beam techniques along with optical profilometry to characterize the porosity of these films. Rutherford backscattering spectroscopy (RBS) and 16O(d,p0)17O nuclear reaction were used to determine the total number of Si and O atoms in the films. Interaction of these films with water was characterized by the 1H(19F, alpha-gamma)16O resonant nuclear reaction. Combination of these techniques provides fast accurate, and quantitative methods for characterizing these films. However, the high-energy ion beams appear to cause significant damage in the films. X-ray photoelectron spectroscopy (XPS) measurements from the ion beam interacted region interacted region show a tail in the low binding energy side of the Si 2p core level spectrum which is characteristic to metal Si. In addition, craters as deep as 100nm were left in the films where the ion beams interacted with the material. We will dicuss the effectiveness of these techniques for these porosity measurements in nanoporous low dielectric-constant silica films.
2000. "Irradiation Effects and Thermal Annealing Behavior in H2(+)-Implanted 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 166-167:374-378. Abstract RBS/Channeling has been used to study the accumulation and isochronal recovery of disorder on the Si sublattice in 6H-SiC single crystals irradiated with 100 keV H₂⁺ ions at 100 and 300 K. The disorder at the damage peak shows a sigmoidal dependence on ion fluence for both irradiation temperatures. Dramatic simultaneous recovery is observed for the irradiation at 300 K. At low fluences, isochronal recovery occurs gradually over a wide temperature range. At high fluences, a near amorphous state is produced, and significant recovery does not occur. At intermediate fluences, damage recovery occurs more rapidly between 300 and 670 K. Further annealing at 1070 K results in the formation of blisters. Hydrogen depth profiles at 100 and 300 K are comparable and are well predicted by SRIM-97 simulations. Hydrogen release of about 30% is observed for SiC irradiated at 100 K and subsequently annealed at 1070 K for 20 min.
2000. "In-situ Ion Channeling Study of Gallium Disorder and Gold Profiles in Au-implanted GaN." Journal of Applied Physics 87(11):7671-7678. Abstract Disorder accumulation and annealing behavior on the Ga sublattice in GaN implanted with 1.0 MeV Au ions (60? off surface normal) at 180 or 300 K have been studied using in-situ Rutherford Backscattering Spectrometry in a <0001>-channeling geometry (RBS/C). Complete amorphization in GaN is attained at 6.0 Au ions/nm₂ and 20 Au ions/nm₂ for irradiation at 180 and 300 K, respectively. A saturation in the Ga disorder at and behind the damage peak was observed at intermediate ion fluences at both 180 and 300 K. No measurable thermal recovery was found at 300 K for full range of damage produced at 180 K. However, distinct epitaxial regrowth in the bulk and Ga reordering at surface occurred after annealing at 870 K. The implanted Au readily diffuses into highly damaged regions at elevated temperatures, and the redistribution of the Au atoms in the implanted GaN varies with the damage profiles. A double-peak Au profile developed with the maxima located in the amorphous surface region and near the Au mean projected range. The result is interpreted as Au atom diffusion into the amorphous regime at surface and trapping at the defects in the crystal structure. This trapping effect is also evidenced in this study by the suppressed recovery of the Au-decorated disorder in GaN.
2000. "Deuterium Channeling Analysis for He(+)-Implanted 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:501-504. Abstract Deuterium ion channeling is applied to study accumulated disorder on Si and C sublattices in 6H-SiC crystals irradiated with 50 keV He(+) ions at 100 and 300 K. The relative disorder on both sublattices follows sigmoidal dependence on dose. Carbon disorder is higher at low doses, suggesting a smaller C displacement energy. Isochronal annealing data show that the recovery behavior on the Si and C sublattices is similar. Annealing of a buried amorphous SiC layer, produced at 100 K, exhibits an epitaxial growth rate of ~0.154 nm/K in the temperature range from 370 to 870 K.
2000. "MOCVD Growth and Structure of Nb- and V-doped TiO2 Films on Sapphire." Journal of Crystal Growth 212 (1-2):178-190. Abstract Describes the results and process for growing Nb- and V-doped TiO2 thin films at a doping level up to 40 at% on sapphire.
2000. "Mechanistic Study of Metalorganic Chemical Vapor Deposition of (Ba,Sr)TiO₃ Thin Films." Journal of Applied Physics 87(10) (May 15 2000):7430-7437. Abstract The metalorganic precursor chemistry was studied on Pt(111) surfaces in an 16 O₂ and 18 O₂ backgrounds. Using temperature programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS). The precursor chemistry of Sr(thd)2 was found to be different on oxide covered Pt(111) surface as compared to the clean Pt(111) surface. In an oxygen ambient, TPD showed at least four different reaction processes which involved the removal of carbon from the precursor ligands on oxide covered Pt(111). In two of these, gas phase oxygen was incorporated in the oxidative products. In contrast, one carbon removing reaction was observed on the clean Pt(111) surface. Isotopic labeling experiments have also been carried out to understand the film-formation reactions in the metalorganic chemical vapor deposition of (Ba,Sr)TiO₃ (BST) films. Time-of-flight SIMS and nuclear reaction analysis (NRA) reveal that the oxygen in the BST films originates from both the gas phase oxidants (18 O) and the precursor ligands (16 O). The ligands substitution by gas phase O₂ plays a more prominent role in the film-formation at lower temperatures. On the other hand, the reactive oxygen radicals produced by microwave plasma involved more in breaking in the O-C bonds than substituting the precursor ligands for the film formation. Use of the 50%18 O2-50%N₂ 16 O mixture results in a reduction of 18 O incorporation in the BST films, indicative of the direct involvement of N₂O in the film-formation reactions. The mechanistic studies are essential for understanding the new BST precursors used in this study, and provide useful information to correlate the film microstructure, step coverage, and dielectric properties with the precursors properties.
2000. "Surface Structure of MBE-Grown Fe3O4(001) by X-Ray Photoelectron Diffraction and Scanning Tunneling Microscopy." Surface Science 450(2000):L273-L279. Abstract We have investigated the surface termination and interlayer relaxations of Fe3O4(001) grown on MgO(001) by oxygen-plasma-assisted molecular beam epitaxy. Despite the fact that autocompensated surfaces can be constructed in principle by terminating with either a half layer of tetrahedral Fe, or a modified layer of octahedral Fe plus tetrahedral O, the combination of photoelectron diffraction and scanning tunneling microscopy suggest the former. The first four interlayer spacings are relaxed by-14%, -57%, -19%, and +29% of the respective bulk value. PACS numbers: 61.10Lx, 68.55.Bd.
2000. "Low Dielectric Constant Mesoporous Silica Films Through Molecularly Templated Synthesis ." Advanced Materials 12(4):291-294. Abstract N/A
1999. "Morphological and Structural Investigation of the Early Stages of Epitaxial Growth of Alpha-Fe2O3 (0001) on Alpha-Al2O3 (0001) by Oxygen-Plasma-Assisted MBE." Surface Science 443(3):212-220. Abstract We have investigated the early stages of Alpha-Fe2O3 (0001) film growth on Alpha-Al2O3 (0001) using oxygen-plasma-enhanced molecular beam epitaxy along with reflection high-energy electron diffraction, noncontact atomic force microscopy, and x-ray photoelectron spectroscopy and diffraction. A compressionally-strained, fully-stoichiometric Alpha-Fe2O3 film three monolayers thick forms prior to the onset of three-dimensional island formation and lattice relaxation. The surface of this film appears to buckle along <1120>, giving rise to a new set of inwardly-contracted diffraction spots which, if not resolved from the substrate spots, could be interpreted as a 12% in-plane lattice parameter expansion. Such an interpretation has led prior investigator to conclude that the interfacial layer consists of a disordered cation layer with an in-plane lattice parameter ~6% larger than that of Alpha-Fe2O3 [4]. Our interpretation of the diffraction data suggests that the interfacial layer is badly distorted, but commensurate with the substrate.
1999. "The Ion Beam Materials Analysis Laboratory at the Environmental Molecular Sciences Laboratory." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 420(1-2):81-89. Abstract Describes the equipment capabilities at EMSL with special emphasis on the accelerator, ion beam analysis and ion beam modification.
1999. "Surface Structure of MBE-Grown alpha-Fe2O3(0001) by Intermediate-Energy X-ray Photoelectron Diffraction." Surface Science 425(2-3):276-286. Abstract Uses intermediate-energy x-ray photoelectron diffraction to determine the surface structure of epitaxial alpha-Fe2O3(0001) grown on alpha-Al2O3(0001) and compares the experiment with quantum mechanical scattering theory. The results agree well with the predictions of molecular mechanics and spin-density functional theory previously reported in the literature for the Fe-terminated surface.
1999. "Rutherford Backscattering and Channeling Studies of Mg and Fe Diffusion at the Interface of gamma-Fe₂O₃(001)/MgO(001)." Surface and Interface Analysis 27(4):194-198. Abstract Investigates the crystalline quality of an epitaxially grown gamma-Fe₂O₃(001) film on Mg/O(001) substrate along with the Mg and Fe inter-diffusion using Rutherford Backscattering and channeling experiments.
1999. "X-ray Photoelectron Diffraction Study of thin Cu Films Grown on Clean Ru(0001) and O-Precovered Ru(0001)." Surface Science 421(3):205-236.
1999. "Growth and Structure of Epitaxial CeO₂ by Oxygen Plasma-Assisted Molecular Beam Epitaxy." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 17(3):926-935. Abstract Investigates the epitaxial growth of CeO₂ films on three different substrates Si(111), SrTiO₃(001) and MgO(001).
1999. "Rutherford Backscattering Spectrometry Channeling Study of Ion-Irradiated 6H-SiC." Surface and Interface Analysis 27(4):179-184. Abstract Studies damage accumulation and defect annealing (up to 1170 K) using in-situ 2.0 MeV He⁺ Rutherford Backscattering Spectrometry combined with ion channeling methods. Observes that the defect concentration at the damage peak increases sigmoidally with increasing ion fluence during irradiation at low temperatures and that the isochronal recovery of the damage induced at low temperatures follows an exponential dependence on temperature.
1999. "Ion-Channeling Study of the SiC/Si/SiO₂/Si Interface." Applied Physics Letters 74(23):3501-3503. Abstract Ion channeling has been used in a detailed study of 3C-SiC films grown by chemical vapor deposition on a Si/SiO₂/Si substrate. For a 160-nm-thick (100)-oriented SiC film, the results show a minimum yield (Xmin) of ~28% at the SiC-Si interface, while a SiC film with a thickness of ~2.4 microns, grown under identical conditions, was almost defect free (Xmin=5.3%) in the surface region. Angular scans around the (110) axis revealed the existence of a superlattice structure at the SiC-Si interface. The strain-induced angular shift ws determined to be 0.16 degree plus or minus 0.05 degrees, indicating a kink between the SiC and Si layers along the inclined (110) axis. A modified model is suggested to interpret the experimental observations.
1999. "Displacement Energy Measurements for Ion-Irradiated 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 148(1-4):557-561.
1999. "Damage Formation and Recovery in C+ Irradiated 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 148(1-4):562-566.
1999. "Epitaxial Growth and Characterization of Ce1-xZrxO2 Thin Films." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 17(3):961-969. Abstract Epitaxial films have been grown on SrTiO3(001) by oxygen-plasma-assisted molecular beam epitaxy. The film growth at 600 degrees C is predominantly nucleation and growth of 3-D islands. The films become much smoother after rapid thermal annealing at 700 degrees C for 30 seconds in the oxygen plasma. High-energy ion channeling reavelas that Zr atoms substitutionally incorporate at cation sites in the CeO2 lattice for all doping levels, leading to Ce1-xZrxO2 solid solutions. Analysis of Zr 3d and Ce 3d core-level binding energies shows that the oxidation state of both Zr and Ce is +4. Lattice distortion induced by incorporation of Zr in the CeO2 lattice beomes prevalent for high doping levels, and surfaces roughen accordingly.
1998. "Damage Accumulation and Annealing in 6H-SiC Irradiated with Si+." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 143(3):333-341. Abstract Damage accumulation and annealing in 6H-silicon carbide (alpha-SiC) single crystals have been studied in situ using 2.0 MeV He⁺RBS in a <0 0 0 1>-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30 degrees off normal) at a temperatur of -110 degrees C, and the damage recovery was investigated by subsequent isochromal annealing (20 min) over the temperature range from -110 degrees C to 900 degress C. At ion fluences below 7.5 X 10 13 Si⁺/cm₂ (0.04 dpa in the damage peak), only point defects appear to be created. Futhermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences of 6.6 x 10 15 Si⁺/cm₂ (-90 degrees C), an amorphous layer is created from the surface to a depth of 0.6 mu-m. Because of recovery processes at the buried crystalline-amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from -90 degrees C to 600 degrees C.
1998. "Accumulation and Recovery of Irradiation Damage in He+ Implanted Alpha-SiC." Journal of Nuclear Materials 257(3):295-302.