2009. "Morphology and Electronic Structure of the Oxide Shell on the Surface of Iron Nanoparticles." Journal of the American Chemical Society 131(25):8824–8832. doi:10.1021/ja900353f Abstract A iron nanoparticle exposed to air at room temperature will be instantly covered by an oxide shell of typical thickness of ~ 3 nm. This native oxide shell in combination with an underlying iron core determines the physical and chemical behavior of this type of core-shell nanoparticles. One of the great challenges for characterizing this type of nanoparticles is determination of the structure of the oxide shell, as it is FeO, Fe3O4, -Fe2O3, -Fe2O3, or anything else. Significant research effort, mostly based on x-ray diffraction and spectroscopy and electron diffraction and transmission electron microscopy imaging, has been made to determine the structure of this thin layer of iron oxide. Most of the experimental results have been framed with one of the known iron oxide structures, although it is not necessarily true that this thin layer of iron oxide consists of a standard iron oxide. In this paper, the structure of the oxide shell on iron nanoparticle is probed using electron energy loss spectroscopy (EELS) at O K-edge with a spatial resolution of several nanometers (individual particle). Two types of representative particles were studied: particles that are fully oxidized and core-shell particle which possesses a Fe core. We found that the O K-edge spectra collected on the oxide shell in the nanoparticles shows distinctive differences as compared with that of the known iron oxide. Based on finger printing and quantum mechanical calculations results, we conclude that the distances between the absorbing oxygen and the next-nearest neighbor oxygens are more widely distributed than that in bulk Fe3O4 for both of these two types of particles. For smaller and fully oxidized particles, there is also a broadened distribution between the absorbing oxygen and the nearest neighbor oxygens. These results clearly demonstrate that the coordination configuration in the oxide shell on Fe nanoparticle is defective as compared with that of their bulk counterpart. Of the two types particles examined in this work, the degree of disorder is larger for the smaller fully oxidized particles.
2009. "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. "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. "Biocompatible core-shell magnetic nanoparticles for cancer treatment." Journal of Applied Physics 103(7):Art. No.07A308. doi:10.1063/1.2831791 Abstract Non-toxic magnetic nanoparticles (MNPs) have expanded the treatment delivery options in the medical world. With a size range from 2 to 200 nm MNPs can be compiled with most of the small cells and tissues in living body. Monodispersive iron-iron oxide core shell nanoparticles were prepared in our novel cluster deposition system. This unique method of preparing the core shell MNPs gives nanoparticles very high magnetic moment. We tested the nontoxicity and uptake of MNPs coated with/without dextrin by incubating them with rat LX-1 small cell lung cancer cells (SCLC). Since core iron enhances the heating effect [7] the rate of oxidation of iron nanoparticles was tested in deionized water at certain time interval. Both coated and noncoated MNPs were successfully uptaken by the cells, indicating that the nanoparticles were not toxic. The stability of MNPs was verified by X-ray diffraction (XRD) scan after 0, 24, 48, 96, 204 hours. Due to the high magnetic moment offered by MNPs produced in our lab, we predict that even in low applied external alternating field desired temperature can be reached in cancer cells in comparison to the commercially available nanoparticles. Moreover, our MNPs do not require additional anti-coagulating agents and provide a cost effective means of treatment with significantly lower dosage in the body in comparison to commercially available nanoparticles.
2008. "Aging of Iron Nanoparticles in Aqueous Solution: Effects on Structure and Reactivity." Journal of Physical Chemistry C 112(7):2286-2293. doi:10.1021/jp0777418 Abstract Aging (or longevity) is one of the most important and potentially limiting factors in the use of nano-Fe0 to reduce groundwater contaminants. We investigated the aging of FeH2 (Toda RNIP-10DS) in water with a focus on changes in (i) the composition and structure of the particles (by XPS, XRD, TEM, and bulk Fe0 content), and (ii) the reactivity of the particles (by carbon tetrachloride reaction kinetics and electrochemical corrosion potentials). Our results show that the FeH2 becomes more reactive between 0 and ~2 days aging, and then gradually loses reactivity over the next few hundred days. These changes in reactivity correlate with evidence for rapid destruction of the original Fe(III) oxide film on FeH2 during immersion and the subsequent formation of a new passivating mixed-valence Fe(II)-Fe(III) oxide shell. The behavior of “unaged” nano-Fe0 in the laboratory may be similar to that in field-scale applications for source-zone treatment due to the short reaction times involved. Long-term aged FeH2 acquires properties that are relatively stable over weeks or even months.
2008. "Controlling Size of Gold Clusters in Polyaniline from Top-Down and from Bottom-Up." Journal of Electroanalytical Chemistry 621(2):238-244. doi:10.1016/j.jelechem.2007.11.025 Abstract Polyaniline forms a strong complex with chloroaurate at the protonated imine sites. Here we report on electrochemical procedure that allows preparation of gold clusters by adding gold atoms one-by-one (“bottom up” approach). It is contrasted with the “top down” approach in which the growth of multi-atom Au clusters was also controlled electrochemically. Our results confirm that both the amount and the size of gold clusters affects the properties of the composite material.
2008. "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. "Focused ion beam directed self-assembly (Cu2O on SrTiO3 ): FIB pit and Cu2O nanodot evolution." Superlattices and Microstructures 44(4-5):677-685. doi:10.1016/j.spmi.2008.01.016 Abstract A gallium focused ion beam has been used to create discrete pits on the surface of a SrTiO3 (100) surface with the idea that these pits will serve as the nucleation sites for subsequent Cu2O quantum dot growth. Immediately after pit formation and following wet chemical etching and thermal annealing of the surface, the concentration of gallium within these pits has been analyzed using a high-resolution Auger system,. Using atomic force microscopy, the geometry of the pits has also been determined following etching and annealing. Growth of Cu2O quantum dots on the patterned surfaces has been performed. Growth of Cu2O quantum dots within the pits is the primary mode of dot formation. In several samples, dot growth within pits appears to occur by a two-step process with pits filling prior to initiation of a second, distinct phase of quantum dot growth above the plane of the original SrTiO3 surface.
2008. "Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates." Thin Solid Films 516(18):6088-6094. doi:10.1016/j.tsf.2007.11.007 Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and ZrO2 (111)-buffered Al2O3 (0001) substrates using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on pure Al2O3(0001) substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3(0001) substrate and the films. However, the films, grown on a thin layer of ZrO2(111) buffered Al2O3 (0001) substrate, appears to be highly oriented. These films were characterized using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS) depth profiling. Oxygen ionic conductivity in gadolinia-doped ceria films was measured as a function of Gd concentration and these results were compared with the ion conductance data of the polycrystalline and single crystalline yttria-stabilized zirconia (YSZ).
2008. "Characterization Challenges for Nanomaterials." Surface and Interface Analysis 40(3-4):529-537. doi:10.1002/sia.2726 Abstract Nanostructured materials are increasingly subject to nearly every type of chemical and physical analysis possible. Because of their small feature size there is a significant focus on tools with high spatial resolution. Because of their high surface area, it is also natural to characterize nanomaterials using tools designed to analyze surfaces. Regardless of the approach, nanostructured materials present a variety of obstacles to adequate, useful and needed analysis. This paper provides short overviews to some of the issues and complications including: particle stability, environmental effects, specimen handling, surface coating, contamination and time. Some specific examples are provided from a our work focused on ceria nanoparticles and iron metal-core/oxide-shell nanoparticles in which we use a combination of tools for routine analysis including XPS, TEM, and XRD and apply other methods as needed to obtain essential information.
2007. "Electrochemical Effects of S Accumulation on Ion-implanted Alloy 22 in 1 M NaCl Solutions." Corrosion Science 49(6):2497-2511. doi:10.1016/j.corsci.2006.12.003 Abstract The objective of this study was to examine the effects of high levels of S in the near-surface region on the passivity of Alloy 22, a corrosion resistant Ni-Cr-Mo alloy, in deaerated 1 M NaCl solution. Near-surface concentrations of S up to 2 at.% were achieved in Alloy 22 test specimens by implanting them with S. The S-implanted samples were then evaluated in short-term electrochemical tests in the salt solution and subsequently analyzed with X-ray Photoelectron Spectroscopy (XPS) for film thickness and composition. Specimens tested included non-implanted and annealed Alloy 22 samples, samples implanted with S, and “blanks” implanted with Ar as an ion that would simulate the “damage” of S implantation without the chemical effect. A sample of S-implanted Alloy 22 was also exposed to solution for 29 days and analyzed for evidence of S accumulation at the surface over longer times
2007. "Morphology and Oxide Shell Structure of Iron Nanoparticles Grown by Sputter-Gas-Aggregation." Nanotechnology 18(25):Art. No. 255603. doi:10.1088/0957-4484/18/25/255603 Abstract Much recent research effort has been made on the synthesis, characterization, and property evaluation of core-shell structured Fe nanoparticles. Fundamental properties of these particles depend on both their external crystal faceting planes and the nature of a protective oxide layer. In this paper, the crystal faceting planes and oxide coating structures of core-shell structured iron/iron oxide nanoparticles synthesized by a sputter-gas-aggregation process were studied using transmission electron microscopy (TEM), electron diffraction and Wulff shape construction. The particles grown by this process and deposited on a support at room temperature process have been compared with particles grown and deposited at high temperature as reported in literature. Most synthesis processes produce round particles for particles less than 20 nm in diameter. For larger particles crystallographic facets are observed. It has been found that the Fe nanoparticles formed at RT are invariantly faceted on the {100} lattice planes and truncated by the {110} planes at different degrees. Substantial fraction of particles are confined only by the 6 {100} planes (not truncated by the {110} planes), this contrasts with the Fe particles formed at high temperature (HT) for which a predominance of {110} planes has been reported. Furthermore, at RT no particle was identified to be only confined by the 12 {110} planes which is relatively common for the particles formed at HT. The Fe cubes defined by the 6 {100} planes show a characteristic inward relaxation along the <100> and <110> directions and the reason for this behavior is not fully understood. The oxide shell on the Fe {100} plane maintains an orientation relationship: Fe(001)//Fe3O4(001) and Fe[100]//Fe3O4[110], which is same as the oxide formed on a bulk Fe(001) through thermal oxidation. Orientation of the oxide that forms on the Fe{110} facets differs from that on Fe{001}, therefore, properties of core-shell structured Fe nanoparticle faceted primarily with one type of lattice plane may be fully different from that faceted with another type of lattice planes.
2007. "Electron Beam Induced Thickening of the Protective Oxide Layer around Fe Nanoparticles ." Ultramicroscopy 108(1):43-51. doi:10.1016/j.ultramic.2007.03.002 Abstract There are many circumstances in science where the process of measuring the properties of a system alters the system. An imaging process can exert an inadvertent effect on the object being observed. Consequently, what we observe does not necessarily represent what had been present before the observation. Normally this effect can be ignored if the consequence of such a change is believed not to be significant. The expansion of nanostructured materials has made high resolution transmission electron microscopy one of the indispensable tools for probing the characteristics of nano-materials. Modification of nanoparticles by the electron beam during their imaging has been widely noticed and this is generally believed to be due to electron beam induced heating effect, defect formation in the particles, charging of the particle, or excitation of surrounding gases. However, an explicit experimental identification of which process dominates is often very hard to establish. We report the thickening of native oxide layer on iron nanoparticle under electron beam irradiation. Based on atomic level imaging, electron diffraction, and computer simulation, we have direct evidence that the protecting oxide layer formed on Fe nanoparticle at room temperature in air continues to grow during an electron beam bombardment in the vacuum system typical of most TEM systems. Partial illumination of a nanoparticle and observation of the shell thickening conclusively demonstrates that many of the mechanisms postulated to explain such processes are not occurring to a significant extent. The observed growth is not related to the electron beam induced heating of the nanoparticle, or residual oxygen ionization, or establishment of an electrical field, rather it is related to electron beam facilitated mass transport across the oxide layer (a defect related process).
2007. "Nucleation and Growth of MOCVD Grown (Cr, Zn)O Films – Uniform Doping vs. Secondary Phase Formation." Journal of the Electrochemical Society 154(3):D134-D138. doi:10.1149/1.2424422 Abstract We report a detailed study of chromium solubility and secondary phase formation in MOCVD grown (Cr, Zn)O-based films on silicon (100). Simultaneous deposition of 0.15M Cr(TMHD) and 0.025M Zn(TMHD) based precursors in an oxidizing environment with a flow ratio of 1:10 resulted in secondary phase formation rather than uniform Cr doping. Based on several surface and micro-structural techniques, we have identified nano-crystalline ZnCr2O4 and disordered Cr2O3 as the secondary Cr-containing phases that nucleate. Analysis suggests that ZnCr2O4 crystallites are dispersed throughout the film and that disordered Cr2O3 layer may form at the interface. These results reveal a strong tendency for Cr to exist in octahedral, rather than tetrahedral coordination.
2007. "Metalorganic chemical vapor deposition of carbon-free ZnO using the bis(2,2,6,6 tetramethyl-3,5-heptanedionato)zinc precursor." Journal of Materials Research 22(5):1230-1234. doi:10.1557/JMR.2007.0146 Abstract We report the growth of c-axis oriented ZnO films on silicon (100) single crystal substrates by MOCVD. A relatively uninvestigated precursor, Zn(TMHD)2, was used in a cold-wall MOCVD reactor. XPS and RBS analysis yielded a zinc-to-oxygen atom percent ratio of 0.98 and 1.00 at the surface and in the overall film, respectively indicative of stoichometric ZnO. Due to the presence of carbon at the surface, the excess oxygen at the surface was in the form of C-O bonding. The c-axis orientation was confirmed by HRTEM and XRD. We look at these results from a viewpoint of an ongoing effort to ensure cleaner decompositions using Zn(TMHD)2.
2007. "Improving Surface Analysis Methods for Characterization of Advanced Materials by development of standards, reference data, and interlaboratory comparisons." Surface and Interface Analysis 39(4):283-293. doi:10.1002/sia.2508 Abstract This paper summarizes the results of two surveys examining current needs for improved analyses of surfaces. Surfaces and interfaces are increasingly important to science and to technologies associated with nanoparticles, nano-structured materials and other complex materials including those associated with information systems and medical or biological applications. Adequate characterization of advanced materials frequently requires application of more than one analysis method along with the need to analyze data in increasingly sophisticated and sometimes interrelated ways. It is useful for both new and experienced analysts to have ready access to best practices for obtaining accurate and useful information from a variety of different analysis tools. The International Organization for Standardization (ISO) Committee TC 201 on Surface Chemical Analysis and ASTM Committee E-42 on Surface Analysis are working to address these needs by assembling guides and standards reflecting the collective experience and wisdom of experts in this community.
2006. "Patterned Regions of Porous Silicon through Trapped Hydrogen Bubbles." Applied Physics Letters 88(14):143113 (3 pages). doi:10.1063/1.2188039 Abstract Hydrogen bubbles trapped around lithographically formed etch pits on silicon single crystal surfaces were used to guide the formation of patterned regions of porous silicon. Optical lithography and wet (electro) chemical etching were utilized in a series of steps to form the etched surface and guide this growth. It was observed that the pits were surrounded by round regions of porous Si suggesting that hydrogen bubbles defined the areas of porous silicon formation. Consistent with earlier results where porosity seemed to form primarily at the bubble edges, a lower overall etch rate was observed near the bubble center. This is consistent with a variable rate of bubble expansion as etching occurred and an ability of the hydrogen to mask or slow the reaction in the bubble center. Contrary to our initial expectations, the surface profile suggested that the region away from the bubbles was also etched, but very uniformly.
2006. "Chemical bonding and electronic structures of the Al2SiO5 polymorphs, andalusite, sillimanite, and kyanite: X-ray photoelectron- and electron energy loss spectroscopy studies." American Mineralogist 91(5-6):740-746. doi:10.2138/am.2006.1887 Abstract We have undertaken a detailed analysis of the X-ray photoelectron spectra obtained from the three polymorphs of Al2SiO5; andalusite, sillimanite, and kyanite. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each polymorph. The spectra for Si(2p) for all three polymorphs are nearly identical, consistent with the fact that all the Si atoms are in 4-fold (tetrahedral) coordination, whereas the binding energies, peak shapes, and peak widths for Al(2p) vary depending on the type of polymorph. The upper-valence band for all three polymorphs is characterized by four main features derived from O(2p), Al(3s), Al(2p), Si(3s), and Si(3p), and the differences in their contributions are observed. The density of state of the Al2SiO5 polymorphs is relatively featureless compared to those observed from α-SiO2 and α-Al2O3, suggesting that the orbital overlaps span a greater range in energy. The observed band gap energy for Al2SiO5 (sillimanite) was ~9.1eV, a value in between those for α-SiO2 (~8.6eV) and α-Al2O3 (~9.6eV). The conduction band feature of Al2SiO5 was experimentally compared to those of α-SiO2 and α-Al2O3, and shown that it is indeed intermediate between the α-SiO2 and α-Al2O3 phases.
2006. "Formation of Cu2O Quantum Dots on SrTiO3 (100): Self-Assembly and Directed Self-Assembly." Journal of Applied Physics 100(9):Art. No. 094315. doi:10.1063/1.2364038 Abstract Nanoscale islands of Cu2O have been synthesized on single-crystal SrTiO3 (100) substrates using oxygen plasma-assisted molecular-beam epitaxy (OPA-MBE). Island growth location has been controlled by using an ex-situ Ga+ focused ion beam (FIB) to modify the growth surface in discrete locations prior to island synthesis. Analysis of Cu2O dot growth on unmodified substrate regions revealed an evolution of dot size and array density. Atomic force microscopy studies show that certain FIB substrate modification and MBE growth condition combinations lead to directed self-assembly of islands. Islands initially formed in the FIB-generated surface topography and filled those features before nucleating on neighboring unmodified surface regions.
2006. "Synthesis and Characterization of Stable Iron–Iron Oxide Core–Shell Nanoclusters for Environmental Applications." Journal of Nanoscience and Nanotechnology 6(2):568-572. doi:10.1166/jnn.2006.074 Abstract Iron–iron oxide core–shell nanoclusters are of great interest due to their potential applications as a remedy for environmental contamination. We report the room-temperature synthesis of core–shell iron–iron oxide nanoclusters using our novel cluster deposition system. Various types of measurements such as Transmission Electron Microscopy, X-ray Diffraction, X-ray Photon Spectroscopy, and Electron Energy Loss Spectroscopy are conducted in characterizing nanoclusters. Stable, monodispersive iron–iron oxide core–shell nanocrystals are identified.
2006. "Size dependent specific surface area of nanoporous film assembled by core-shell iron nanoclusters." Journal of Nanomaterials 2006(Special Issue 2):Art. No. 54961. doi:10.1155/JNM/2006/54961 Abstract Nanoporous films of core-shell iron nanoclusters have improved possibilities for remediation, chemical reactivity rate and environmentally favorable reaction pathways. Conventional methods often have difficulties to yield stable monodispersed core-shell nanoclusters. We produced core-shell clusters by a cluster source that utilizes combination of Fe target sputtering along with gas aggregations in an inert atmosphere at 7ºC. Sizes of core-shell iron-iron oxide nanoclusters are observed with transmission electron microscopy (TEM). The specific surface areas of the porous films obtained from Brunauer-Emmett-Teller (BET) process is size dependent and compared with the calculated data.
2005. "Void Formation during Early Stages of Passivation: Initial Oxidation of Iron Nanoparticles at Room Temperature." Journal of Applied Physics 98(9):094308(7). doi:10.1063/1.2130890 Abstract The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some uncertainties. Exposure of most clean metals to air or oxygen results in the nearly instant formation of an oxide layer. Because this initial layer normally forms in a relatively uncontrollable manner, the atomic level understanding of the initial oxidation is limited in comparison to the abundant experimental observation and theoretical derivation on thickening of oxidation layer on metal surface at high temperature. We report in this letter mMicrostructurale characterization of iron nanoparticles oxide passivated with iron oxide shell nanoparticleswere studied using high resolution transmission electron microscopy (HRTEM) and high angle annular dark-filed (HAADF) imaging in aberration corrected scanning transmission electron microcopy (STEM). Voids were readily observed on both small single crystal -Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticle has been engineered for Co at higher temperatures1, they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some controversy. For example, void formation has been noticed in the single crystal -Fe nanoparticles as a consequence of iron outward diffusion during the initial oxidation at room temperature. There exists a critical size of ~ 8 nm for which the iron has been fully oxidized, leading to a hollow iron oxide nanoparticle. For particles larger than the critical size, an iron/iron oxide core-shell structure was formed and voids reside at the interface between the oxide shell and the iron core. The present observation provides new insight for tailoring of metal/metal-oxide core-shell structured nanoparticles for applications related to optics, magnetism, and nanoelectronics.
2005. "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. "Gold-polyaniline composites: Part II. Effects of nanometer sized particles." Physical Chemistry Chemical Physics. PCCP 7(20):3619-3625. Abstract The amount of electronic charge transferred between gold particles and polyaniline depends not only on the electron affinity of the two materials but also on the size of the gold particles. As measured by X-ray photoelectron spectroscopy, for particles < 5 nm the binding energy of the electrons is size dependent. This nano-effect has its origin in the electrostatics of particles. It is demonstrated as a measurable shift of the binding energy of the Au4f7/2 photoelectrons emitted from Au particles embedded in a polyaniline matrix. Gold nanoparticle size was evaluated by high resolution transmission electron microscopy.
2005. "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. "Characterization and Properties of Metallic Iron Nanoparticles: Spectroscopy, Electrochemistry, and Kinetics." Environmental Science and Technology 39(5):1221-1230. Abstract There are reports that nano-sized zero-valent iron (Fe⁰) exhibits greater reactivity than micro-sized particles of Fe⁰, which may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. To better understand the reactivity of iron nanoparticles, we have used a variety of complementary techniques to characterize two widely studied nano Fe⁰ preparations: one synthesized by heat-reduction of goethite under H₂ (FeH₂) and the other by reductive precipitation with borohydride (FeBH). X-ray diffraction (XRD), transmission electron microscopy (STXM) showed particles of similar size (40-80 nm), but surface area measurements varied widely with method of measurement (4-60 m² g-¹). FeH₂ is a two-phase material consisting of ⍺-Fe⁰ and Fe₃O₄, doped with reduced sulfur, whereas FeBH is mostly metallic Fe with an oxide shell that is high in boron. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe₂O₃, Fe₃O₄, micro-sized Fe⁰, or a solid Fe⁰ disk, consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinone-which presumably probes inner-sphere surface reactions-reacts more rapidly with FeBH than with FeH₂, whereas with carbon tetrachloride, FeBH and FeH₂ react at similar rates, presumably by outer-sphere electron transfer. Whether either material reacts more rapidly with the probes than micro-sized Fe⁰ is unclear due to uncertainties in the appropriate specific surface areas. The distribution of products from reduction of carbon tetrachloride is more favorable with FeH₂, which produces less chloroform than reaction with FeBH.
2005. "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. "Erosion Rate Variations during XPS Sputter Depth Profiling of Nanoporous Films." Surface and Interface Analysis 37(4):417-423. Abstract Sputter depth profiling is commonly used to obtain valuable information regarding the three dimensional distribution of elements within a sample, and is one of the best ways to measure the composition of a buried interface or the uniformity of a thin film. X-ray photoelectron spectroscopy (XPS) is one of the analysis tools often used in conjunction with ion beam erosion to obtain sputter depth profiles. However, to obtain accurate depth information it is often necessary to better understand the sputtering process for a specific materials system. Artifacts such as differential sputtering, varying sputter rates and ion beam-induced chemistry are well known. Here, however, we present evidence from experiments on a porous thin film deposited on a Si wafer that relatively small chemical and/or structural changes in a nanoporous film can affect the rate of erosion measured during sputter depth profiling. Reproducible variations in sputter rate are found with chemical modification leading to compositional changes of the nanoporous thin film. The origin of the sputter rate changes is discussed with the aid of results obtained using Fourier transform infrared spectroscopy, profilometry, nuclear reaction analysis, electron microscopy and XPS-based depth profiling.
2005. "Summary: Update to ASTM Guide E 1523 to Charge Control and Charge Referencing Techniques in X-ray Photoelectron Spectroscopy." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 23(3):577-578. Abstract An updated version of the ASTM guide E1523 to the methods to charge control and charge referencing techniques in x-ray photoelectron spectroscopy has been released by ASTM. The guide is meant to acquaint x-ray photoelectron spectroscopy (XPS) users with the various charge control and charge referencing techniques that are and have been used in the acquisition and interpretation of XPS data from surfaces of insulating specimens. The current guide has been expanded to include new references as well as recommendations for reporting information on charge control and charge referencing. The previous version of the document had been published in 1997.
2005. "Summary of ISO/TC 201 Standard: XVIII, ISO 19318: 2004 – Surface Chemical Analysis – X-Ray Photoelectron Spectroscopy - Reporting of Methods Used for Charge Control and Charge Correction." Surface and Interface Analysis 37(5):524–526. Abstract X-ray photoelectron spectroscopy (XPS) is widely used for characterization of surfaces of materials. Elements in the sample (with the exception of hydrogen and helium) are identified from comparisons of the binding energies of their core levels, determined from measured photoelectron spectra, with tabulated values of these binding energies for the various elements. Information on the chemical state of the detected elements can frequently be obtained from small variations (typically between 0.1 eV and 10 eV) of the core-level binding energies from the corresponding values for the pure elements. Reliable determination of chemical shifts often requires that the binding-energy scale of the XPS instrument be calibrated with an uncertainty that could be as small as 0.1 eV. The surface potential of an insulating specimen will generally change during an XPS measurement due to surface charging, and it is then difficult to determine binding energies with the accuracy needed for elemental identification or chemical-state determination. There are two steps in dealing with this problem. First, experimental steps can be taken to minimize the amount of surface charging (charge-control methods). Second, corrections for the effects of surface charging can be made after acquisition of the XPS data (charge-correction methods). Although the buildup of surface charge can complicate analysis in some circumstances, it can be creatively used as a tool to gain information about a specimen.
2005. "Simple Method for Estimating and Comparing of X-Ray Damage Rates." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 23(6):1740-1744. doi:10.1116/1.2073387 Abstract This note describes an approach for estimating and comparing rates or thresholds for x-ray induced specimen damage during surface analysis. The method uses a common reference material to compare x-ray damage rates reported in the literature and in publications to the rates of damage that may occur on a specific instrument. Although the method makes several assumptions that are only partially valid, results from a few damage data-sets appear acceptably consistent when comparing estimates to a time for 10% damage or signal change.
2005. "Challenges in Applying Surface Analysis Methods to Nanoparticles and Nanostructured Materials." Journal of Surface Analysis 12(2):101-108. Abstract Nanostructured materials of various types and forms are formulated in a variety of novel ways and increasingly subject to many types of chemical and physical analysis. Since nanomaterial systems contain a relatively large amount of surface or interface area, it should be natural to characterize them using tools designed to analyze surfaces and interfaces. We have found that nanoparticles and other nanostructured materials present a variety of challenges. This paper reviews environmental effects on measurements of Ce-oxide nanoparticles and nanoporous silica films and focuses on efforts to quantify the ion damage and sputter rates for the Fe-oxide nanoparticles. We have found that nanoparticles appear more readily damaged and to have sputter rates that exceed “bulk” materials. To verify such effects, we need to know many details about size, size distribution, density, and shape that are not always easily obtained.
2004. "Dissolution and Growth of (10(1) over-bar4) Calcite in Flowing Water: Estimation of Back Reaction Rates via Kinetic Monte Carlo Simulations." Journal of Crystal Growth 262(1-4):503-518. Abstract Although calcite is an important mineral for many processes, there ae been relatively few simulations of it's growth and dissolution behavior. Such simulations are complicated by the multitude of defect types and by the asymmetry of the crystal. The present work combined a kinetic Monte Carlo (KMC) technique with the Kossel crystal (100) simple cubic concept and the Blasius boundary layer model to simulate the simultaneous growth and dissoution of the (1014)calcite cleavage surface in flowing water. The objective was to determine the activation energies of the back reaction (growth) from those of the forward reaction (dissolution) by obtaining agreement with cleavage-step morphologies and step dissolution velocities previously measured using an atomic force microscope (AFM). Blasius boundary layer conditions for the flowing fluid defined a model that treated the solid, the dissolution/growth interface, and the fluid kinetics. Microscopic reversibility and the laws of large numbers gave an expression for the back reaction activation energies in terms of the forward reaction energies and the entropy of mixing, a quantity estimated from the concentration of desorbates in a very small fluid layer adjacent to the interface. The KMC simulations produced cleavage-step morphologies that were in qualitive agreement with observations from AFM. The kinetics were dominated by diffusion events on the solid/fluid interface and in the fluid, as expected. The relative magniyudes of the desorption and adsorption activation energies were consistent with experimental data, entropic arguments, and crystal roughening theories. Quantitive agrrement with measured step velocities was best when the boundary layer parameters were given physically reasonable values, indicating that the model is self consistent.
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. "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. "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. "Focused-Ion-Beam Directed Self-Assembly of Cu₂O Islands on SrTiO3(100)." Applied Physics Letters 84(25):5213-5215. Abstract Nanoscale islands of Cu₂O have been synthesized on single crystal SrTiO₃ (100) substrates using oxygen plasma assisted molecular beam epitaxy (MBE). Island growth location has been controlled by using an ex-situ Ga⁺ focused ion beam (FIB) to modify the growth surface in discrete locations prior to island sythesis. The FIB modifications have generated surface topography with lateral dimensions of 150-200 nm. Ex-situ AFM study after island growth reveals that certain FIB substrate modification and MBE growth condition combinations lead to directed self-assembly of metal oxide islands at the edges of the FIB modified zones.
2003. "Beam Effects During AES and XPS Analysis." Chapter 9 in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy. IM Publications, Chichester, United Kingdom. Abstract It is important to realize that any surface analysis method may alter the specimen in some way. Alterations that complicate the ability to collect the desired information are usually considered damage. Damage (like beauty) is in the eye of the beholder. In some cases, analysis-induced changes to a sample will have little or no impact on the information sought. In other cases, similar changes will be totally unacceptable and considered information-destroying damage. The analyst must therefore be able to recognize damage in all its various forms, understand its origins, and be able to compensate for, or limit, its effects on the analysis.
2003. "Appendix F: Comparing Beam Damage Rates Using Susceptibility Tables." In Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, ed. D. Briggs and J.T. Grant, pp. 845-856. SurfaceSpectra Ltd. & IM Publications, Manchester, United Kingdom. Abstract Many research groups have observed electron and x-ray damage on a variety of materials. It is, of course, highly desirable to take advantage of the considerable experience gained by others in observing the presence of damage. Ideally, damage processes and rates would be determined for the samples of interest on the instrument in which measurements are to be made. However, in many cases such measurements are not possible and many important speciments are one of a kind. Several compilations of damage rates are available in the literature 1,2, or from Companies 3. In most cases the data from these potentially useful data sets are not likely to directly relate to the damage rates that might be observed for other instruments. There are two related reasons for this. First there is over an order of magnitude difference between x-ray damage rates observed on instruments in current use, as reported by Yoshihara and Tanaka.4 In addition, many new instruments have higher x-ray fluxes than many older instruments. The damage rates observed in the Pacific Northwest National Laboratory (PNNL) PHI Quantum 2000, are in the lower 1/3 of the instruments reported in Reference 4 but are approximately 5 times faster than those reported by Beamson and Briggs1 in 1992 for the Scienta 300 system.
2003. "Temperature-Induced Phase Separation in Chromium Films." Applied Physics Letters 82(14):2230-2232. Abstract Study of phase seperton at Cr/Si interface. Vacuum evaporation of chromium on Si(100) results in an interfacial nanophase layer followed by a mixed grain region. In this work, the mixed grain region in chromium is nearly transformed into a single-phase region, which resulted in a maximum phase separation between nano -A15 phase and bulk -bcc chromium. It is achieved by creating a temperature gradient across chromium-silicon interface by using rapid evaporation at a growth rate of ~3 m/min at a source-tosubstrate (SS) distance of 2 cm. Separately-grown nanophase chromium films at SS distance of 32 cm show predominant -A15 phase. Short-SS-distance rapid evaporation has a potential to produce selective phase separation by combining temperature gradient with the interfacial stress.
2003. "Investigation of Copper(I) Oxide Quantum Dots by Near-Edge X-ray Absorption Fine Structure Spectroscopy." Chemistry of Materials 20(15):3939-3946. Abstract Copper oxide quantum dots (CuOQD) were grown in various thicknesses on different SrTiO₃(001) surfaces and were investigated by near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The experimental growth conditions for the CuOQD were optimized to obtain Cu₂O as the major phase. The CuOQD grown on clean SrTiO₃(001) surfaces at 825 K or higher with p(O₂) of 9.0x10-7 Torr or above contain mostly CuO contrasting to CuOQD grown at 800 K with p(O₂) of ~7.0x10-7 Torr that contain primarily Cu₂O. Furthermore, it is established that there is a strong interaction between the SrTiO₃(001) surface and the first few monolayers of the CuOQD, which induces the formation of Cu(II). However, this interaction is mitigated with increasing thickness of CuOQD resulting in the exclusive formation of Cu₂O in the topmost layers. The influence of the SrTiO₃(001) substrate on the formation of CuOQD can be reduced by modifying the substrate surface using chemical treatment and/or energetic Au₂⁺ ion-beam irradiation, since the substrate effect results from the reaction between the substrate oxygen and the copper atoms from the CuOQD. Examination of the photochemical properties of these CuOQD shows that prolonged soft x-ray irradiation under vacuum reduces Cu(II), which is present as a minor impurity in the CuOQD.
2003. "Heteroepitaxial Growth of a Manganese Carbonate secondary Nano-Phase on the (101_4) Surface of Calcite in Solution." Surface Science 524(1-3):63-77. Abstract Heteroepitaxy of a manganese carbonate phase with nanometer dimensions on the (101_4) surface of calcite (CaCO₃) using an AFM has been observed in solution during dissolution of calcite when the ion activity product of Mn₂⁺ and CO₃₂- nears the solubility limit of MnCO₃. Growth-rate observations at different Mn concentrations, coupled with XPS and EPR measurements, suggest that the resulting phase is Mn₀․₅Ca₀․₅CO₃. These islands, while growing many microns in length along the [22_1] direction, have a uniform width in the range of 120-240 nm and a uniform height of approximately 2.7 nm, corresponding to nine atomic layers. The islands cease growing when they encounter step edges and have been observed to dissolve when undercut by a growing etch pit.
2003. "Electron Beam Damage in Poly(Vinyl Chloride) and Poly(Acrylonitrile) as Observed by Auger Electron Spectroscopy." Surface Science Spectra 10:67-79. Abstract AES spectra of spun-cast films of poly(vinyl chloride) (PVC) and poly(acrylonitrile) (PAN) were collected over a period of time to determine specimen damage during exposure to a 10kV electron beam. For the PVC, loss of chlorine was observed over a period of 203 minutes to the extent that the final chlorine concentration was only 20% of its original value. PAN exhibited a loss in nitrogen content over a period of 120 minutes, but the rate of damage to the polymer was significantly less than PVC. Figure 1 shows the atomic concentration in the PVC film as a function of dose (time). It takes a dose of approximately 7.0x10-5 Ccm-5 for the chlorine concentration to fall from its original value by 10% (one definition of critical dose). Figure 2 shows a similar drop in nitrogen concentration in the PAN film as a function of dose. For this polymer, it takes a dose of 1.3x10-3 Ccm-2 for the nitrogen concentration to fall by 10%.
2003. "Beam Damage of Poly(Vinyl Chloride) [PVC] Film as Observed by X-ray Photoelectron Spectroscopy." Surface Science Spectra 10(101):57-66. Abstract XPS spectra of a spin-coated film poly(vinyl chloride) (PVC) were collected over a period of 243 minutes at 303 K to determine specimen damage during long exposures to monochromatic Al Ka x-rays. For this PVC film we measured the loss of chlorine as a function of time by rastering a focused 104.6 w 100 um diameter x-ray beam over a 1.4 mm x 0.2 mm area on the sample.
2003. "Beam damage of poly(vinyl chloride) [PVC] as observed by x-ray photoelectron spectroscopy at 143 K, 303 K and 373 K." Surface Science Spectra 10:101-116. Abstract X-ray beam damage is often observed during surface analysis of beam sensitive materials as indicated in the introduction to this issue and in a wide variety of references. While damage occurs in a wide range of materials, those that are most susceptible to damage are materials that contain low energy covalent bonds such as polymers or other organic materials. Even amongst the relatively easily damaged polymers, there is a wide range of damage rates. The data reported in this submission was collected in the interest of comparing the rates of damage produced by x-ray and electron irradiation of different materials. The focus of this submission is x-ray damage of bulk poly(vinyl chloride) PVC since it is a readily available material. The temperature of the sample holder was controlled during irradiation of the PVC in order to determine the extent to which localized sample heating affects the rate of damage. PVC degrades by photoionization, resulting in the production of HCl through H and Cl bond cleavage. Bulk PVC has previously been the focus of an x-ray damage study involving many laboratories throughout the world. For comparison to the bulk PVC data, damage rates for thin films of poly(2-chloroethyl methacrylate) (PCEMA) are also reported for the same three temperatures. Measurements of several relatively common materials on one instrument can provide a data base that allows damage rates on one instrument to be linked or compared to other damage data in the literature.3 , , , In addition to the PCEMA films and bulk PVC, other data collected at the same x-ray parameters include thin films of PVC (for which damage rates are essentially identical to the bulk material) and of poly(acrylonitrile) PAN which is more stable that either PCEMA or PVC. An additional set of PVC data for a different instrument is also included in the volume. X-ray beams used for XPS produce less damage in materials than electrons used for AES analysis. This is due to both the weaker interaction between x-rays and materials (relative to electrons) and the deeper penetration of x-rays into the material producing a lower damage density. Although the rates of damage for x-rays and electrons differ, many of the processes are similar. Both electron and x-ray damage has been collected for PVC and PAN as reported in this volume of SSS8,9,12 and summarized in the introduction. The overall damage rates for PVC and PAN differ by an order of magnitude, but the ratios of the electron and x-ray damage rates for these two materials are nearly the same.1,7
2003. "Beam damage of poly(2-chloroethyl methylacrylate) [PCEMA] films as observed by x-ray photoelectron spectroscopy at 143 K, 303 K, and 373 K." Surface Science Spectra 10:80-100. Abstract X-ray beam damage is often observed during surface analysis of beam sensitive materials as indicated in the introduction to this issue and in a wide variety of references. While damage occurs in a wide range of materials, those that are most susceptible to damage are materials that contain low energy covalent bonds such as polymers or other organic materials. Even amongst the relatively easily damaged polymers, there is a wide range of damage rates. The focus of this submission is on poly(2-chloroethyl methylacrylate) [PCEMA] films. In order to determine the extent to which localized sample heating could influence damage rates the temperature of the substrate holding the PECMA was controlled during irradiation. PCEMA presumably degrades primarily by photo-ionization, resulting in the production of HCl through H and Cl bond cleavage. PCEMA has been recommended as a polymer for use as a reference for evaluating x-ray damage. PCEMA has been shown to be more sensitive to degradation than PVC which has also been used as a damage sensitive material useful for comparison of damage rates. Measurements of several relatively common materials on one instrument can provide a data base that allows damage rates on one instrument to be linked or compared to other damage data in the literature.3,4,5, Therefore for purposes of comparison, damage rates for bulk PVC at the same three different temperatures used for the PCEMA data have been collected and are also presented in this volume. Other data collected at the same x-ray parameters include thin films of PVC (for which damage rates are essentially identical to the bulk material) and of poly(acrylonitrile) PAN which is more stable that either PCEMA or PVC.
2003. "Sharpening our Understanding but Blurring the Boundaries: Dynamic Observations of Surface Reconstruction ." Surface Science 540(2-3):153-156. Abstract Every now and then, reading a specific paper stimulates - in my mind at least - a variety of associations and connections that highlight advances that have been made and suggests links between areas that I may not have previously connected. The recent series of papers by McCarty and Bartelt (and co-workers) using low energy electron microscopy (LEEM) to study the dynamics of surface reconstruction of TiO₂ , and NiAl sent my thinking in a variety loosely connected directions. Paraphrasing the response of one of my colleagues - the work causes us to think dynamically where we have often thought statically about what happens when surfaces reconstruct. The measurements also highlight the importance of newer techniques to help us visualize and understand phenomena that may have puzzled us for years. The dynamic interactions between surface structure and both the defect structure (and history) of the substrate and the nature of the environment of the specimen highlight an aspect of phenomena that drive surface reconstruction not normally considered and suggests additional and delightful challenges we face in understanding the “bulk†stability and surface structures of nano-sized objects. Since the physical arrangement of the atoms controls every aspect of the physics and chemistry of a surface or interface, the atomic geometry is a fundamental defining characteristic of a surface. , Details of the structure of a surface, including altered atomic positions, the presence of steps and various types of defects can significantly change the chemistry of a surface and impact processes ranging from the formation of interfaces in electronic components to the efficiency of a catalyst. Because of its importance there has been considerable effort devoted to understanding and predicting surface structures. However, dynamical aspects of surface reconstruction and the significance of material defects in the process have not been part of the standard picture.
2003. "Introduction to Surface Science Spectra Data on Electron and X-ray Damage: Sample Degradation during XPS and AES Measurements." Surface Science Spectra 10(1-4):47-56. Abstract The types of damage to sample surfaces that can occur during X-ray or electron irradiation used during Auger electron spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) are summarized. The material and instrumental dependence of the rate and nature of damage formation or sample degradation are highlighted. Particular note is made of an enhanced susceptibility of thin films to damage. A simple method to enable comparison of published damage rates to what may be observed in a specific system is described and discussed in relation to data included in this journal volume. Strategies for detecting and minimizing damage are presented.
2003. "Enhancing Coating Functionality Using Nanoscience and Nanotechnology." Progress in Organic Coatings 47:342-356. Abstract Nanoscience and nanotechnology offer a set of tools and new approaches for creating coating with enhanced and optimized properties. The ultimate impact of nanoscience and nanotechnology in many of the potential application areas will depend on our ability to direct the assembly of hierarchal systems that include nanostructures. Two fundamentally different approaches to direct self-assembly are discussed. One approach involves ordering existing identifiable components into the desired coating or structures. The second approach involves the formation of new structures during the coating process. Potential impacts of nanostructure properties on film characteristics and applications are discussed with a focus on coating reactivity, corrosion resistance, strength and durability.
2002. "XAS and XPS Characterization of Monolayers Derived from a Dithiol and Structurally Related Disulfide-Containing Polyamides." Langmuir 18(21):8123-8128. Abstract X-ray absorption spectroscopy and X-ray photoemission spectroscopy have been used to examine sulfur-gold bond formation in self-assembled monolayers derived from a dithiol monomer and disulfide-containing polyamide. These compounds were designed to allow the molecules to adsorb to gold through two terminal sulfurs, forming surface-attached loops. Element and site-specific density of unoccupied electronic states was probed by x-ray absorption spectroscopy at the C₁s, N₁s, O₁s (K-edge) and S 2p (L₂,3-edge) absorption edges. Photoemission measurements of the C₁s, N₁s, O₁s and S 2p core lines were also used to estimate relative coverage, confirm layer formation and evaluate chemical bonding of the monomer and polymer to the Au coated substrates. In case of the dithiol monomer SAM, the spectroscopic evidence clearly shows that most of the molecules adsorb through only one sulfur. The disulfide-containing polymer, in contrast to the monomer, attaches to the surface through both sulfurs to form the anticipated surface-attached loop.
2002. "High Energy Ion Beam Studies of Ion Exchange in a Na₂O-Al₂O₃-SiO₂ Glass." Journal of Applied Physics 91(4):1910-1920. Abstract As part of understanding the processes leading to sodium release and ion exchange, the surface and near surface reaction regions on several specimens of a Na₂O-Al₂O₃-SiO₂ glass have been examined after exposures to isotopicaly labeled aqueous solutions. The majority of the analyses describe here have been carried out using energetic ion beam analysis. Rutherford backscattering spectrometry (RBS) has been used to measure the overall glass composition and to determine the profiles and amounts of Na released from the surface. An important part of the ion exchange process is the uptake and incorporation of hydrogen and oxygen in the glass from the solution. To facilitate this analysis, the glasses were exposed to a solution containing 18O and deuterium and analyzed by accelerator based nuclear reaction analysis (NRA). To confirm some of the RBS depth profile data very near the surface, XPS depth profiles were collected on some samples. Although the Na concentration is decreased in the near surface region, it is not totally removed from the outer surface. In this same region, there is also a significant amount of 18O incorporated demonsrating considerable interaction between the water and the glass. Deeper into the material the amounts of deuterium and 18O are more consistent with water or H3O+ diffusion. These results suggest that there exist an outer reaction layer and an inner diffusion controlled layer in the surface region of the reacted glass.
2002. "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. "Electronic Structure of Ytterbium-Doped Strontium Fluoroapatite: Photoemission and Photoabsorption Investigation ." Journal of Applied Physics 91(8):5135-5140. Abstract X-ray photoemission and x-ray photoabsorption were used to study the composition and the electronic structure of ytterbium-doped strontium fluoroapatite (Yb:S-FAP). High resolution photoemission measurements on the valence band electronic structure and Sr 3d, P 2p and 2s, Yb 4d and 4p, F 1s and O 1s core lines were used to evaluate the surface and near surface chemistry of this fluoroapatite. Element specific density of unoccupied electronic states in Yb:S-FAP were probed by x-ray absorption spectroscopy (XAS) at the Yb 4d (N4,5-edge), Sr 3d (M4,5-edge), P 2p (L2,3-edge), F 1s and O 1s (K-edges) absorption edges. These results provide the first measurements of the electronic structure and surface chemistry of this material.
2002. "Evidence for Localization of Reaction Upon Reduction of Carbon Tetrachloride by Granular Iron." Langmuir 18(20):7688-7693. Abstract The distribution of reaction sites on iron particles exposed to water containing carbon tetrachloride has been examined by measuring the locations of reaction products. The uniformity or localization of reaction sites has implications for understanding and modeling the reduction of environmental contaminants by iron in ground water systems. Granular iron surfaces similar to those being used for environmental remediation applications were studied using surfaces analysis techniques to develop an understanding of the physical and chemical structure of the surface and oxide films. Scanning Auger microscopy and imaging time-of-flight secondary ion mass spectrometry revealed that granular iron exposed to carbon tetrachloride-saturated water exhibits chloride-enriched regions occurred at pits rather than on th passive oxide film on the metal. Understanding the nature of the local solute reduction sites will play an important role in modeling the kinetics of reaction at passive iron oxide films in environmental systems.
2002. "Hydration of Passive Oxide Films on Aluminum." Journal of Physical Chemistry B 106(18):4705-4713. Abstract Models for the corrosion and pitting of passive metals such as aluminum usually involve the migration of point defects through the native oxide film as the rate limiting step. Hydration of the surface oxide could also influence the protective nature of the film. Secondary ion mass spectrometry (SIMS) has been used in conjunction with isotopic labeling to determine the extent and rate of passive film hydration on aluminum. The rates at which oxygen- and hydrogen-contianing species migrate through the film has been determined as a function of temperature and applied potential (cathodic and anodic polarization). The results suggest that defects such as hyroxide ions are prevalent and mobile in the oxide film, influencing the kinetics and mechanisms of corrosion processes.
2002. "Use and Limitations of Electron Flood Gun Control of Surface Potential During XPS: Two Non-homogeneous Sample Types." Surface and Interface Analysis 33:781-790. Abstract The ability of charge compensation methods to control the surface potentials for two types of non-homogenous samples is examined. Results demonstrate that two newer types of charge compensation systems have improved performance in relation to some previous flood gun methods and reaffirm the concept that a primary objective of charge compensation is to find conditions for which the surface potential of the specimen is as uniform as possible. However, experiments involving both flood gun use and specimen grounding, demonstrate that peak broadening and shifting can occur when two (or more) potentials are present in the region of analysis. Finally, the ability of interface charge to shift specimen potentials and measured binding energies demonstrates fundamental limitations to the absolute accuracy of binding energy measurements, but also remind us that charging phenomena can be used to obtain important information about the sample.
2002. "Practical Aspects of Charging Phenomena in XPS as demonstrated in Oxidized-Al Films on Al and Al Alloys." Journal of Surface Analysis 9(3):396-403. Abstract Understanding the surface and interface structure, composition and chemistry of insulating materials has long been of importance in surface analysis. The relevance of insulating surfaces to the environment, and the increasing use of newer and more complex oxide films in electronic and optical applications significantly increases the information needed about these materials. Through a series of examples using Al-oxide films, this paper summarizes some conceptual and practical issues related to analysis of insulators, including vacuum-level and Fermi-level referencing, charge buildup at interfaces, the use of charge neutralization, the impact of electron and ion-beam damage, and the influence of impurities on oxide properties and measurements. Many of the measurements are understood through consideration of potential variation through a specimen. Current results, along with many in the literature, demonstrate that surface charging is not a problem that can be readily solved and ignored, but is a tool providing important information about materials and films.
2001. "Analyzing Localized Corrosion in Ion-Implanted Metals via XPS/AES." JOM. The Journal of the Minerals, Metals and Materials Society 53(7):37-41. Abstract Intergranular Stress Corrosion Cracking (IGSCC) of metals is often controlled by anodic (oxidative) dissolution of the metal at the crack tip. The anodic processes, in turn, depend on both crack-tip solution and grain boundary compositions. The latter are strongly influenced by alloying constituents and impurities that frequently segregate to (or away from) the grain boundaries during heat treatment and/or aging. For systems that exhibit anodic control, understanding the kinetics of the corrosion processes at crack tips and, in particular, determining how the kinetics are influenced by the presence and amount of grain boundary segregants, is critical to identifying both promising andproblematic alloys and in developing predictive crack growth models.
2001. "Scanning Auger Microscopy Studies of an Ancient Bronze." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 19(4 PT 1):1126-1133. Abstract We have conducted a scanning Auger microscopy (SAM) analysis on a Syrian bronze of the early I millennium BC. The objective was to derive a picture of modifications of the object's surface composition, induced by chemical attack by the ambient. Understanding degradation of ancient materials dating from a well-defined historical context provides information about long-term corrosion phenomena that is not possible from laboratory data. Such information is also useful for restoration and conservation efforts. While corrosion processes in the bulk are understood, this is not true for localized intergranular and transgranular surface corrosion in these materials. This requires information about local chemical composition and movement of ionic species that has not generally been available, and we find AES useful in accomplishing this task. SAM images of Cu, Sn and O, ~ 100-nm spatial resolution, and Auger point spectra show that Sn acts as a barrier against O2 attack, as it entirely traps this gas into a SnO2-like oxide, thus minimizing copper oxidation. Sub-micron spectromicroscopic evidence is given that S-induced corrosion occurs mainly via chemical attack along grain boundaries, where Sn has a low abundance and copper sulfides are detected. Lateral segregation of Sn and Cu domains is imaged with a spatial resolution of 15 nm. This result marks the best spatial resolution any analytical method has yet achieved in highlighting lateral chemical heterogeneities of ancient bronzes. Although archaeomaterials lie outside the mainstream applications of Auger techniques, this study provides convincing evidence that SAM can greatly advance our understanding of these materials.
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. "Microscopic Effects of Carbonate, Manganese, and Strontium ions on Calcite Dissolution." Geochimica et Cosmochimica Acta 65 (3):369-379. Abstract Aqueous dissolution of the (1014) surface of calcite was observed at pH near 9 using an atomic force microscope equipped with a fluid cell. The influence of carbonate, Sr, and Mn ion concentrations were observed. Carbonste ions were shown to have a step-specific effect on calcite dissolution. At ow levels (5 mu-M) of carbonate, the retreat rate of the more structually open [441]+steps was than the retreat rate of the structurally confined [441]-steps, leading to anisotropic dissolution. Increasing the carbonate level to 200 mu-M decreased the rate of retreat of both steps, but the [411]+step was slowed to a much greater extent making the dissolution nearly isotropic. At high levels (800 mu-M) of carbonate, the rate of retreat of the [441]+step was slower than that of the [441]-step making dissolution anisotropic in the opposite sense to that observed at low levels of carbonate. This decrease in step velocity at high carbonate levels was attributed to a corresponding increase in the reaction (i.e., precipitation) as the solution approached saturation with respect to calcite, and thus is related to the rate of incorporation of calcium cations into the structure. In addition to changing the rate, this back reaction also altered the shape of etch pits formed by dissolution. Strontium cations were also shown to have a step-specific effect on calcite dissolution similar to that of carbonate, suggesting that strontium is preferentially incorporated into the [441]-step to a greater extent than strontium. When the solution exceeded saturation with respect to rhodochrosite, calcite dissolution was nearly isotropic. These results suggest that the small manganese ion (r = 83 pm), is readily incorporated into both [441]+ and [441]-steps, in contrast to the larger Ca (r = 100 pm) and Sr (r = 131 pm) cations, which are preferentially incorporated into the [441]+step.
2001. "Role of Mg in the Stress Corrosion Cracking of an Al-Mg Alloy." Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing 32(7):1699-1711. Abstract The corrosion and stress corrosion cracking (SCC) susceptibility of an Al-Mg alloy, AA5083, has been shown to depend on the precipitation of the Mg rich b phase, (Al3Mg2), but not the enrichment of elemental Mg at grain boundaries to an enrichment ratio (ER) of 1.4. These results were determined by measuring the progress of Mg enrichment at grain boundaries, for increasing thermal treatment times, using Auger Electron Spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by Analytical Electron Microscopy (AEM) of thin foils. The progress of the b-phase precipitation was followed by AEM and Scanning Electron Microscopy (SEM), for the same thermal treatment times. The lack of a Mg segregation effect on SCC was demonstrated by results obtained with X-Ray Photoelectron Spectroscopy (XPS) analysis of Mg implanted Al following in situ electrochemical tests and SCC tests while the dominance of b-phase precipitation was demonstrated by electrochemical analysis and SCC testing. Crack growth tests of alloy AA5083 demonstrated faster cracking at potentials anodic to the open circuit potential (OCP) with no increase at potentials cathodic to the OCP.
2001. "Role of Magnesium in the Stress Corrosion Cracking of an Al-Mg Alloy ." Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science 32A:1699. Abstract The corrosion and stress corrosion cracking (SCC) susceptibility of an Al-Mg alloy has been shown to depend on the precipitation of the Mg rich phase, Al3Mg2, but not the enrichment of elemental Mg at grain boundaries to concentrations up to 3 times that of the alloy. These results were determined by measuring the progress of Mg enrichment at grain boundaries, for increasing thermal treatment times, using Auger Electron Spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by Analytical Electron Microscopy (AEM) of thin foils. The progess of the Al3Mg2 precipitation was followed by AEM and Scanning Electron Microscopy (SEM), for the same thermal treatment times. The lack of a Mg segregation effect was demonstrated by both X-Ray Photoelectron Spectroscopy (XPS) analysis of Mg implanted Al following in situ electrochemical tests (1) and SCC testing while the dominance of Al3Mg2 precipitation was demonstrated by electrochemical analysis and SCC testing. Crack growth tests of alloy AA5083 conducted at potentials cathodic and slightly anodic potentials with no increase at cathodic potentials. This is consistent with reported hydrogen permeation results for Al which suggests a hydrogen contribution to crack growth.
2001. "The interplay between step anisotropy and surface phase transformation on TiO2(110)." Physical Review. B, Condensed Matter 63(12):1401. Abstract The dynamic behavior of steps on TiO2(110) was studied during the transformation from the (1x1) to the (1x2) surface. It was found that two types of steps dominate through the process: the steps run along the [110] direction (A-type) and the one parallel to the [110](B-type). The stoichiometric (1x1) surface contained mostly A-steps. Reducing of this surface caused nearly half of the A-steps to be gradually converted to the B-steps. The evolving of the step structure was accompanied by the growth of the reconstructed (1x2) domains initiated at the B-step edges. On the (1x2) domains, approximately equal length of A- and B-steps were identified. The results shed light on the step structure and correlation between the step energetics and suface phase transformations on the TiO2(110) surface.
2001. "Effects of titania surface structure on the nucleation and growth of Pt nanoclusters on rutile TiO2(110)." Surface Science 475(1-3):159-170. Abstract A comparative study was conducted on the interaction of Pt with the (1x1) and (1x2) surface phases of rutile TiO2(110). It was found that the surface structure and stoichiometry of TiO2 have profound effects on the formation and growth of nano-sized Pt clusters on TiO2(110). On the (1x1) surface, Pt formed randomly-distributed, three-dimensional nanoclusters that coalesced and were encapsulated when thermally annealed. In contrast, smaller Pt clusters were identified to adsorb on top of the titanium atomic rows on the (1x2) surface and exhibited higher thermal stability. The stronger interaciton observed for the Pt on the (1x2) surface is due to a charge transfer from Ti to Pt atoms. Vicinal TiO2(110) surfaces with alternating (1x1) and (1x2) domains were used to verify the different interactions of Pt for the two domains. Upon Pt deposition, an array of self-organized Pt nanoclusters was formed on this suface. The self-organization is due to different interactions of Pt with the two different surface domains and preferential diffusion of Pt nanoclusters along the titanium atomic rows on the (1x2) surface.
2001. "Effect of Platinum Nanocluster Size and Titania Surface Structure upon CO Surface Chemistry on Platinum-Supported TiO2 (110)." Journal of Physical Chemistry B 105(12):2412-2416. Abstract The adsorption chemistry of CO on clean and Pt-supported TiO2 (110) was investigated. It was found that surface structure of TiO2 plays an important role in the chemistry that takes place at the surface. On the reduced (1 x 2)-reconstructed surface, CO desorbed at 140 and 170 K, while only desorption at 140 K was observed on the stoichiometric (1 x 1) surface. Additionally, CO dissociation, possibly due to the reduction by Ti 3+ , was observed on the Pt-supported (1 x 2) surface. On the Pt-covered surfaces, the chemistry of CO adsorption and desorption strongly depends on the size of Pt nanoclusters. With a decrease in cluster size, CO was found to desorb at higher temperatures. This unusual desorption chemistry is likely related to quantum size effects of Pt nanoclusters. Scanning tunneling spectra revealed that clusters below 20 ? in diameter exhibited nonmetallic behavior, while those above 40 ? were metallic. This transition of the properties of Pt nanoclusters from metallic to nonmetallic as the cluster size decreases correlates with stronger interaction of CO with Pt observed in temperature-programmed desorption spectra.
2000. "Atomic Control of TiO2(110) Surface By Oxygen Plasma Treatment." Surface Science Letters 459(3):L498-L502. Abstract Segregation of impurities in TiO2 substrates impacts the local atomic structure and stoichiometry of the native TiO2 surfaces. We investigated the effects of oxygen plasma on the TiO2 surfaces. It was found that annealing TiO2 crystals at above 500 degrees C effectively removed calcium and carbon contaminants and dramatically improved the surface morphology and stoichiometry. Scanning tunneling micrographs revealed atomically smooth surfaces with well-defined step-terrace morphology after the plasma treatment, while x-ray photoelectron spectra showed that all the Ti cations on the resulting surfaces were fully coordinated.
2000. "Third Row Transition Metals by X-ray Photoelectron Spectroscopy." Surface Science Spectra 7(1):1-68. Abstract XPS Spectrum have been obtained from eleven metals in the third row of transition elements using a Quantum 2000 Scanning ESCA Microprobe. The metals analyzed include La, Hf, Ta, W, Re, Ir, Pt, Au, Tl, Pb, and Bi. Each sample was Ar> {+} ion etched before XPS analysis to remove any surface contamination and/or oxide. The spectrum include standard survey scans and high energy resolution scans of the photoelectron peaks, as well as selected x-ray induced Auger peaks. Each spectrum was collected using a 100 um monochromatic Al Ka x-ray beam scanned over a 1.5 mm x 0.2 mm area of each sample. Survey scans were collected using an 58.7 eV pass energy, while high energy resolution scans were collected using a 23.5 eV pass energy.
2000. "Influence of Mg on the Corrosion of Al ." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 18(1):131-136. Abstract This paper summarizes a series of experiments to determine the influence of Mg on the corrosion and electrochemical behavior of Al. Magnesium is commonly added to increase the strength of lightweight non-heat treatable Al alloys. However, these alloys are susceptible to grain boundary dissolution, stress corrosion cracking or hydrogen induced embrittlement due to changes in the alloy structure and the elemental distribution during processing,welding, or in-service exposure to elevated temperatures. Auger electron spectroscopy and transmission electron microscopy measurement show that alloys having a distributuion of Al3Mg2 (beta phase) precipitates and segregated Mg on grain boundaries are more susceptible to cracking. To understand the roles of Mg on the cracking process we compared the corrosion potential and film formation of pure AL, AL implanted with Mg, a 7 wt% Mg-Al alloy and pure Al3Mg2 phase. The surfaces of the specimens were cleaned and prepared in a surface analysis system and transferred in a vacuum transfer system to a corrosion cell. After solution exposure and electrochemical measurement the specimens were returned to the spectrometer and analyzed by x-ray photoelectron spectroscopy. The open circuit potentials for Al, Mg implanted Al, and the 7% alloy were nearly identical. However, the corrosion potential for the beta phase differs significantly. The thickness of the film formed on each of the samples is similar. Mg is observed to be depleted in the outer part of the oxide films, but somewhat enhanced near the oxide-metal interface. The results suggest that segregrated Mg plays little role in the cracking and that hydrogen production of the Beta phase particles may be the most significant factor.
2000. "Approach for Determining Area Selectivity in Small-Area XPS Analysis." Surface and Interface Analysis 29(11):766-772. Abstract This paper demonstrates the measurements on well defined "dots" of a material on a substrate provide a useful experimental approach for determining the area of a speciman that contributes to a small area XPS measurement. The method provides information that can verify instrument operation conditions and the adequacy of speciman alignment procedures. A data set collected primarily on one system demonstrates how some general properties of an instrument can be learned in order to understand and optimize data collection methods. The relationship of common methods of measuring spatial resolution and the dot test method are discussed.
1999. "Corrosion of Phosphorus-Implanted 304L Stainless Steel in 1 N H2SO4." Journal of the Electrochemical Society 146(3):984-987. Abstract Combined electrochemical and surface analysis measurements revealed that 304L stainless steel, implanted with P, oxidizes at near-transpassive anodic potential in 1 N H2SO4 to give corrosion products that affect the corrosion rate differently depending on the amount of P at the surface of the metal. At low concentrations, P assisted corrosion by facilitating dissolution of the oxidation products, while at high concentrations, the products were insoluble and precipitated to form a thick, electrically resistive film. The resulstant voltage drop contributed to lower currents as long as the thick film was stable. The dependence of corrosion rate on P concentration is believed to contribute to the "two-peak" current repsonse boserved for the material in potential-step measurements.
1999. "Interactions of Liquid and Vapor Water with Stoichiometric and Defective TiO2(100) Surfaces." Surface Science 440(1-2):60-68. Abstract Interactions of both liquids and vapor water with stoichiometric (nearly defect-free) and defective TiO2(100) surfaces have been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). For an almost defect-free (100) 1X1 surface, water coverage was ~0.08ML (1 ML=7.36x10 4/cm2) at 10 4 L exposure to low-vapor-pressure water, ~0.32ML at 10 8 L exposure to higher-vapor-pressure water, and ~0.50 ML with liquid-water exposure, respectively. Defect intensities were greatly reduced after exposing defective surfaces to ~10 2 L low-vapor-pressure water. More significantly, electron-beam-induced defects were completely removed upon higher exposure (>10 4 L), while defects created by Ar+ bombardment were partially removed. The surface structural influence on defect reactivity has been examined by comparing the results obtained from both (100) and (110) surfaces. Defects on (100) surfaces were removed more readily than those on (110) surfaces.
1999. "The Ion Beam Materials Analysis Laboratory at the Environmental Molecular Sciences Laboratory." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 420(1-2):81-89. Abstract Describes the equipment capabilities at EMSL with special emphasis on the accelerator, ion beam analysis and ion beam modification.
1999. "Effect of Oxidation State of Palladium in Polyaniline Layers on Sensitivity to Hydrogen." Electroanalysis 11(10-11):774-781. Abstract It is shown that the ratio of Pd 0/Pd2+ in polyaniline (PANI) has a major effect on the response of film to hydrogen. The palladium clusters were incorporated in PANI as the result of spontaneous reduction of PdSO4 by electroactive PANI in the leucoemeraldine or nigraline form, respectively. The two forms of PANI were prepared, by terminating the electrochemical deposition at -0.1 V (PANI -0.1V) and by terminating it at +0.7 V (PANI 0.7V). A higher amount of Pd is incorporated in the in PANI when the eduction of Pd2+ is initiated from the leucoemeraldine form. The ratio of the amine/imine units correlates well with the changes of the Pd 0/Pd2+ ratio as shown by the XPS. The changes of the work function were measured by a Kelvin Probe. A substantial decrease of WF was observed when the PANI -0.1V -Pd composite film was composed to hydrogen. On the other hand the WF of PANI 0.7V-Pd film increases upon exposure to hydrogen. The change in the electron donor/electron acceptor behavior towards hydrogen is governed by the PANI/Pd matrix.
1999. "Electrochemical Formation of Au Clusters in Polyaniline." Chemistry of Materials 11(10):2989-2994. Abstract The reduction of chloroaurate and the incorporation of Au clusters in polyaniline films (PANI) has been investigated. The chloroaurate complex is generated at the electrode surface during Cl doping of Au/PANI. FTIR and UV/Vis data indicate that chloroaurate interacts with PANI and that its reduction to metallic Au occurs preferentially at the nitrogen linkages. The voltammetric and XPS results show that the uptake of both protons and anions is suppressed by the formation of Au clusters due to this interaction. The ability to reduce chloroaurate in PANI films is also demonstrated for Pt electrodes coated with PANI in solutions containing KAuCl4. The preliminary results indicate that Au cluster size distribution remains fairly constant regardless of method used.
1999. "Vacuum Cleaved Calcium Carbonate by XPS." Surface Science Spectra 6:153-159. Abstract Calcite is a common mineral phase found in pure and mixed phase systems. The data reported here was collected for comparison with data collected on other systems and to determine the relative sensitivity for various Ca photopeaks that we were using for the analysis of other Ca-containing mineral phases. This data was collected on a PHI Quantum Scanning ESCA Microprobe. This instrument is unique in that the x-ray beam is focused and can be scanned over the sample. A single crystal of calcite was broken in vaccum (exposing the 1014 cleavage plane) and analyzed using monochromatic Al K alpha x-rays. The sample was neutralized using both low energy ions and electrons.