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The Glenn T. Seaborg Institute for Transactinium Science: 1997-1998 activity report

Description: The Glenn T. Seaborg Institute for Transactinium Science (GTS-ITS) mission supports the long-term manpower and core competency needs of the defense-related and the environmental programs at LLNL. The importance of the GTS-ITS mission is intensified by the current and impending shortage of actinide experts. This sector continues to retire, leaving ever-widening gaps in the core capabilities of the programs and disciplines throughout the Laboratory and the nation. The GTS-ITS focus is to help the Laboratory fill these voids by educating and training the future generation of scientists with the knowledge and expertise required to meet the nation's changing needs in environmental protection and remediation, nuclear waste isolation, national security nuclear surveillance, nuclear energy and industrial applications of nuclear methods, and fundamental transactinium sciences.
Date: June 1, 1999
Creator: Terminello, L.J.
Partner: UNT Libraries Government Documents Department

Photoelectron holography of platinum (111)

Description: Platinum atoms near a (111) single-crystal face have been imaged using photoelectron holography. Electron angular intensity patterns were collected at equally spaced wavenumbers from 6 to 12{Angstrom}{sup {minus}1}. Images of atoms near expected atomic positions are obtained from single-wavenumber analyses over the range of the data set. Positions are detected further from the emitter than we have seen previously, and symmetry assumptions are not required. We have also adopted a three dimensional means of representing the data in order to help understand the results. Twin image suppression and artifact reduction in the holographically reconstructed data are set are obtained when images at different wavenumbers are correctly phase-summed. We are assessing the capability of the technique for rendering true three-dimensional structural information for unknown systems.
Date: April 1, 1993
Creator: Petersen, B.L.; Terminello, L.J.; Barton, J.J. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

X-ray absorption studies of uranium sorption on mineral substrates

Description: Uranium L{sub 3}-edge x-ray absorption spectra have been measured for uranium-mineral sorption systems. An expansible layer silicate, vermiculite, was treated to obtain a collapsed and non-expanding phase, thereby limiting access to the interior cation exchange sites. Samples were prepared by exposing the finely powdered mineral, in the natural and modified form, to aqueous solutions of uranyl chloride. EXAFS spectra of the encapsulated samples were measured at the Stanford Synchrotron Radiation Laboratory. Results indicate that the uranyl ion possesses a more symmetric local structure within the interlayer regions of vermiculite than on the external surfaces.
Date: November 1994
Creator: Hudson, E. A.; Terminello, L. J. & Viani, B. E.
Partner: UNT Libraries Government Documents Department

Occupied and unoccupied orbitals of C{sub 60} and C{sub 70}

Description: The full spectrum of occupied and unoccupied {sigma} and {pi} orbitals is presented for solid C{sub 60}, C{sub 70}, and graphite, using Cls emission and absorption spectroscopy. There are significant differences between C{sub 60} and C{sub 70}, and even larger changes relative to their infinite analog graphite C{sub {infinity}}. A comparison is made with photoemission and inverse photoemission results, along with first principles quasiparticle calculations.
Date: March 8, 1994
Creator: Carlisle, J. A.; Terminello, L. J. & Hudson, E. S.
Partner: UNT Libraries Government Documents Department

X-ray absorption study of pulsed laser deposited boron nitride films

Description: B and N K-edge x-ray absorption spectroscopy measurements have been performed on three BN thin films grown on Si substrates using ion- assisted pulsed laser deposition. Comparison of the films` spectra to those of several single-phase BN powder standards shows that the films consist primarily of sp{sup 2} bonds. Other features in the films`s spectra suggest the presence of secondary phases, possibly cubic or rhombohedral BN. Films grown at higher deposition rates and higher ion-beam voltages are found to be more disordered, in agreement with previous work.
Date: February 2, 1994
Creator: Chaiken, A.; Terminello, L. J.; Wong, J.; Doll, G. L. & Sato, T.
Partner: UNT Libraries Government Documents Department

Characterization of the rare earth orthophosphates and Ce-doped LaPO{sub 4} by X-ray Absorption Spectroscopy

Description: X-ray Absorption Spectroscopy (XAS) of the Rare Earth (RE) 3d levels yields sharp peaks near the edges as a result of strong, quasi-atomic 3d{sup 10}4f {yields} 3d{sup 9}4f{sup n+1} (M{sub 4,5}) transitions that contain a wealth of spectroscopic features. XAS is a useful technique for the characterization of 4f-occupancy, 4f-hybridization, and valence in RE-containing materials. The XAS measurements of the single crystal RE-orthophosphates, as well as a range of Cedoped ({approx}1--30%) LaPO{sub 4} hosts were performed at the 3d edge in the total electron yield mode at beamline 8-2 at the Stanford Synchrotron Radiation Laboratory (SSRL). The XAS spectra of the RE ions in the orthophosphate matrix generally resemble that of the corresponding RE metal and emphasize the major contribution of the trivalent state to the electronic transitions occurring at the 3d edge. There is no energy shift of the La and Ce absorption peaks with Ce doping and furthermore, no additional transitions are observed in either spectral region. However, accompanying the Ce doping there is a significant narrowing of the La absorption peak full width half maximum that contrasts to the Ce features that exhibit no contraction. The La and Ce spectra indicate that the Ce-doping of LaPO{sub 4} is purely substitutional.
Date: December 31, 1993
Creator: Shuh, D. K.; Perry, D.; Terminello, L. J.; Boatner, L. A. & Abraham, M. M.
Partner: UNT Libraries Government Documents Department

Molecular Limits to the Quantum Confinement Model in Diamond Clusters

Description: The electronic structure of monodisperse, hydrogen-passivated diamond clusters in the gas phase has been studied with x-ray absorption spectroscopy. The data show that the bulk-related unoccupied states do not exhibit any quantum confinement. Additionally, density of states below the bulk absorption edge appears, consisting of features correlated to CH and CH{sub 2} hydrogen surface termination, resulting in an effective red shift of the lowest unoccupied states. The results contradict the commonly used and very successful quantum confinement model for semiconductors which predicts increasing band edge blue shifts with decreasing particle size. Our findings indicate that in the ultimate size limit for nanocrystals a more molecular description is necessary.
Date: April 7, 2005
Creator: Willey, T M; Bostedt, C; van Buuren, T; Dahl, J E; Liu, S E; Carlson, R K et al.
Partner: UNT Libraries Government Documents Department

Experimental Observation of Quantum Confinement in the Conduction Band of CdSe Quantum Dots

Description: Recent theoretical descriptions as to the magnitude of effect that quantum confinement has on he conduction band (CB) of CdSe quantum dots (QD) have been conflicting. In this manuscript, we experimentally identify quantum confinement effects in the CB of CdSe QDs for the first time. Using X-ray absorption spectroscopy, we have unambiguously witnessed the CB minimum shift to higher energy with decreasing particle size and have been able to compare these results to recent theories. Our experiments have been able to identify which theories correctly describe the CB states in CdSe QDs. In particular, our experiments suggest that multiple theories describe the shifts in the CB of CdSe QDs and are not mutually exclusive.
Date: December 15, 2006
Creator: Lee, J I; Meulenberg, R W; Hanif, K M; Mattoussi, H; Klepeis, J E; Terminello, L J et al.
Partner: UNT Libraries Government Documents Department

Electronic structure of KD2xH2(1-x)PO4 studied by soft x-ray absorption and emission spectroscopies

Description: The surface and bulk electronic structure of tetragonal (at 300 K) and orthorhombic (at 77 K) KD{sub 2x}H{sub 2(1-x)}PO{sub 4} single crystals (so-called KDP and DKDP), with a deuteration degree x of 0.0, 0.3, and 0.6, is studied by soft x-ray absorption near-edge structure (XANES) and non-resonant soft x-ray emission (XES) spectroscopies. High-resolution O K-edge, P L{sub 2,3}-edge, and K L{sub 2,3}-edge XANES and XES spectra reveal that the element-specific partial density of states in the conduction and valence bands is essentially independent of deuteration x. We give assignment of XANES and XES peaks based on previous molecular orbital and band-structure calculations. Projected densities of states in the conduction band also appear to be essentially identical for tetragonal (at 300 K) and orthorhombic (at 77 K) phases, consistent with previous band structure calculations. However, a decrease in sample temperature from 300 to 77 K results in an {approx} 0.5 eV shift in the valence band edge (probed by XES), with negligible changes to the conduction band edge (probed by XANES). Results also show that high-intensity x-ray irradiation results in decomposition of these hydrogen-bonded materials into water and KPO{sub 3} cyclo- and polyphosphates.
Date: April 27, 2004
Creator: Kucheyev, S O; Bostedt, C F; van Buuren, T; Willey, T M; Land, T A; Terminello, L J et al.
Partner: UNT Libraries Government Documents Department

X-ray photoemission electron microscopy for the study of semiconductor materials

Description: Photoemission Electron Microscopy (PEEM) using X-rays is a novel combination of two established materials analysis techniques--PEEM using UV light, and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. This combination allows the study of elemental composition and bonding structure of the sample by NEXAFS spectroscopy with a high spatial resolution given by the microscope. A simple, two lens, 10 kV operation voltage PEEM has been used at the Stanford Synchrotron Radiation Laboratory and at the Advanced Light Source (ALS) in Berkeley to study various problems including materials of interest for the semiconductor industry. In the present paper the authors give a short overview over the method and the instrument which was used, and describe in detail a number of applications. These applications include the study of the different phases of titanium disilicide, various phases of boron nitride, and the analysis of small particles. A brief outlook is given on possible new fields of application of the PEEM technique, and the development of new PEEM instruments.
Date: March 1998
Creator: Anders, S.; Stammler, T.; Padmore, H.; Terminello, L. J.; Jankowski, A. F.; Stohr, J. et al.
Partner: UNT Libraries Government Documents Department

Bonding and hardness in nonhydrogenated carbon films with moderate sp(3) content

Description: Amorphous carbon films with an s p{sup 3} content up to 25% and a negligible amount of hydrogen have been grown by evaporation of graphite and concurrent Ar{sup +} ion bombardment. The s p{sup 3} content is maximized for Ar{sup +} energies between 200 and 300 eV following a subplantation mechanism. Higher ion energies deteriorate the film due to sputtering and heating processes. The hardness of the films increases in the optimal assisting range from 8 to 18 GPa, and is explained by the crosslinking of graphitic planes through s p {sup 3} connecting site.
Date: April 17, 2000
Partner: UNT Libraries Government Documents Department

Composition and Bonding in Amorphous Carbon Films Grown by Ion Beam Assisted Deposition: Influence of the Assistance Voltage

Description: Amorphous carbon films have been grown by evaporation of graphite with concurrent Ar+ ions bombardment assistance. The ion energy has been varied between 0-800 V while keeping a constant ion to carbon atom arrival ratio. Film composition and density were determined by ion scattering techniques (RBS and ERDA), indicating a negligible hydrogen content and a density dependence with the assistance voltage. The bonding structure of the films has been studied by Raman and X-ray Absorption Near-Edge (XANES) spectroscopy. Different qualitative effects have been found depending on the ion energy range. For ion energies below 300 eV, there is a densification of the carbon layer due to the increase in the sp3 content. For ion energies above 300 eV sputtering phenomena dominate over densification, and thinner films are found with increasing assistance voltage until no film is grown over 600 V. The films with the highest SP3 content are grown with intermediate energies between 200-300 V.
Date: November 12, 1998
Creator: Albella, J.M.; Banks, J.C.; Climent-Font, A.; Doyle, B.L.; Gago, R.; Jimenez, I. et al.
Partner: UNT Libraries Government Documents Department

Holographic atom imaging from experimental photoelectron angular distribution patterns

Description: One of the most challenging areas of materials research is the imaging of technologically relevant materials with microscopic and atomic-scale resolution. As part of the development of these methods, near-surface atoms in single crystals were imaged using core-level photoelectron holograms. The angle-dependent electron diffraction patterns that constitute an electron hologram were two-dimensionally transformed to create a three dimensional, real-space image of the neighboring scattering atoms. They have made use of a multiple-wavenumber, phased-summing method to improve the atom imaging capabilities of experimental photoelectron holography using the Cu(001) and Pt(111) prototype systems. These studies are performed to evaluate the potential of holographic atom imaging methods as structural probes of unknown materials.
Date: November 1, 1993
Creator: Terminello, L. J.; Lapiano-Smith, D. A.; Barton, J. J.; Petersen, B. L. & Shirley, D. A.
Partner: UNT Libraries Government Documents Department

Toward a surface photoelectron diffractometer: A progress report on selected advances and an assessment

Description: X-ray diffractometric determination of atomic structures in ordered bulk systems is highly automated and has wide application. By contrast, surface crystallography, whether based on photon or electron scattering, is still in a relatively early stage of development. A summary is given of recent selected highlights in efforts to make progress toward surface photoelectron diffractometry and holography by our Berkeley-Penn State group. It is concluded that an automated photoelectron diffractometer is practical and desirable.
Date: October 6, 1993
Creator: Shirley, D. A; Zheng, Yu; Petersen, B. L.; Hussain, Z.; Huff, W. A.; Barton, J. J. et al.
Partner: UNT Libraries Government Documents Department

Near-Edge X-ray Absorption Fine Structure Spectroscopy of Diamondoid Thiol Monolayers on Gold

Description: Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface-modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 {+-} 0.05 eV and 0.16 {+-} 0.04 eV respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different amounts of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond clusters.
Date: November 27, 2007
Creator: Willey, T M; Fabbri, J; Lee, J I; Schreiner, P; Fokin, A A; Tkachenko, B A et al.
Partner: UNT Libraries Government Documents Department

User-Friendly End Station at the ALS for Nanostructure Characterization

Description: This is a construction project for an end station at the ALS, which is optimized for measuring NEXAFS of nanostructures with fluorescence detection. Compared to the usual electron yield detection, fluorescence is able to probe buried structures and is sensitive to dilute species, such as nanostructures supported on a substrate. Since the quantum yield for fluorescence is 10{sup -4}-10{sup -5} times smaller than for electrons in the soft x-ray regime, such an end station requires bright undulator beamlines at the ALS. In order to optimize the setup for a wide range of applications, two end stations were built: (1) A simple, mobile chamber with efficient photon detection (>10{sup 4} times the solid angle collection of fluorescence spectrographs) and a built-in magnet for MCD measurements at EPU beamlines (Fig. 1 left). It allows rapid mapping the electronic states of nanostructures (nanocrystals, nanowires, tailored magnetic materials, buried interfaces, biologically-functionalized surfaces). It was used with BL 8.0 (linear polarized undulator) and BL 4.0 (variable polarization). (2) A sophisticated, stationary end station operating at Beamline 8.0 (Fig. 1 right). It contains an array of surface characterization instruments and a micro-focus capability for scanning across graded samples (wedges for thickness variation, stoichiometry gradients, and general variations of the sample preparation conditions for optimizing nanostructures).
Date: July 5, 2006
Creator: Himpsel, F. J.; Alivisatos, P.; Callcott, T.; Carlisle, J.; Denlinger, J. D.; Eastman, D. E. et al.
Partner: UNT Libraries Government Documents Department

Soft x-ray emission studies of the electronic structure in silicon nanoclusters

Description: Density of states changes in the valence and conduction band of silicon nanoclusters were monitored using soft x-ray emission and absorption spectroscopy as a function of cluster size. a progressive increase in the valence band edge toward lower energy is found fro clusters with decreasing diameters. A similar but smaller shift is observed in the near-edge x-ray absorption data of the silicon nanoclusters.
Date: February 1, 1997
Creator: Van Buuren, T.; Dinh, L.N.; Chase, L.L.; Siekhaus, W.J.; Jumenez, I.; Terminello, L.J. et al.
Partner: UNT Libraries Government Documents Department

X-ray Raman scattering in H-BN observed by soft x-ray fluorescence spectroscopy

Description: Raman scattering of soft x-rays is observed in h-BN using monochromatic soft x-rays just below the B K absorption edge. The inelastic features are visible below threshold, track with the excitation energy, go through a resonance as the excitation is tuned to the B ls core exciton energy, and finally evolve into normal fluorescence as the excitation is raised above the energy needed to excite the B ls electron into the conduction band. The inelastic energy loss is identified as an excitation of valence {sigma} electrons into the {pi}* valence exciton state; at resonance and above, {pi} {minus} {pi}* transitions are also observed. At resonance, a sideband on the elastic peak Ls observed, which gives evidence of additional electronic and phonon loss processes. Very similar results have also been observed for B{sub 2}O{sub 3}.
Date: March 1, 1995
Creator: Jia, J.J.; Callcott, T.A.; Carlisle, J.A.; Terminello, L.J.; Asfaw, A.; Ederer, D.L. et al.
Partner: UNT Libraries Government Documents Department

Resonant soft x-ray fluorescence studies of novel materials

Description: The authors are using resonant soft x-ray fluorescence at the Advanced Light Source to probe the electronic and geometric structure of novel materials. In the resonant process, a core electron is excited by a photon whose energy is near the core binding energy. In this energy regime the absorption and emission processes are coupled, and this coupling manifests itself in several ways. In boron nitride (BN), the resonant emission spectra reflect the influence of a ``spectator`` electron in an unoccupied excitonic state. The resonant emission can be used to distinguish between the various bulk phases of BN, and can also be used to probe the electronic structure of a monolayer of BN buried in a bulk environment, where it is inaccessible to electron spectroscopies. For highly-oriented pyrolytic graphite (HOPG) a coherent absorption-emission process takes place in the resonant regime, whereby crystalline momentum is conserved between the core excited electron and the valence hole which remains after emission.
Date: February 8, 1995
Creator: Carlisle, J.A.; Terminello, L.J.; Hudson, E.A.; Shirley, E.L.; Jia, J.J.; Callcott, T.A. et al.
Partner: UNT Libraries Government Documents Department