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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

Crystal structure of Thermotoga maritima TM0439: implications for the mechanism of bacterial GntR transcription regulators with Zn2+-binding FCD domains

Description: The GntR superfamily of dimeric transcription factors, with more than 6200 members encoded in bacterial genomes, are characterized by N-terminal winged helix (WH) DNA-binding domains and diverse C-terminal, regulatory domains, which provide a basis for the classification of the constituent families. The largest of these families, FadR, contains nearly 3000 proteins with all a-helical regulatory domains classified into two related Pfam families: FadR{_}C and FCD. Only two crystal structures of the FadR family members, i.e. the E. coli FadR protein and the LldR from C. glutamicum, have been described to date in literature. Here we describe the crystal structure of TM0439, a GntR regulator with an FCD domain, found in the Thermotoga maritima genome. The FCD domain is similar to that of the LldR regulator, and contains a buried metal binding site. Using atomic absorption spectroscopy and Trp fluorescence, we show that the recombinant protein contains bound Ni{sup 2+} ions, but it is able to bind Zn{sup 2+} with K{sub D} < 70 nM . We conclude that Zn{sup 2+} is the likely physiological metal, where it may perform either or both structural and regulatory roles. Finally, we compare the TM0439 structure to two other FadR family structures recently deposited by Structural Genomics consortia. The results call for a revision in the classification of the FadR family of transcription factors.
Date: June 6, 2009
Creator: Zheng, Meiying; Cooper, David; Grossoehmerb, Nickolas; Yu, Minmin; Hung, Li-Wei; Cieslik, Murcin et al.
Partner: UNT Libraries Government Documents Department

On The Electronic Configuration in Pu

Description: X-Ray Absorption Spectroscopy (XAS) and Photoelectron Spectroscopy (PES) have been performed upon highly radioactive samples, particularly Plutonium, at the Advanced Light Source in Berkeley, CA, USA. First results from alpha and delta Plutonium are reported as well as a detailed analysis of sample quality.
Date: January 29, 2007
Creator: Tobin, J G; Soderlind, P; Landa, A; Moore, K T; Schwartz, A J; Chung, B W et al.
Partner: UNT Libraries Government Documents Department

Soft X-ray Studies of Pu Electronic Structure: Past Lessons and Future Directions

Description: Photoelectron Spectroscopy (PES) and X-ray Absorption Spectroscopy (XAS, Figure 1) have contributed greatly to our improved understanding of Pu electronic structure. From these and related measurements, the following has been determined: (1) The Pu 5f spin-orbit splitting is large; (2) The number of Pu5f electrons is near 5; and (3) The Pu 5f spin-orbit splitting effect dominates 5f itineracy. Significant questions remain concerning the nature of Pu electronic structure. Perhaps the missing piece of the puzzle is the direct experimental determination of the unoccupied electronic structure using high energy inverse photoelectron spectroscopy or Bremstrahlung Isochromat Spectroscopy (BIS). Past BIS studies of Th and U indicate the feasibility and utility of Pu studies.
Date: February 7, 2008
Creator: Tobin, J. G. & Yu, S. W.
Partner: UNT Libraries Government Documents Department

Growth and characterization of superconducting spinel oxide LiTi2O4 thin films

Description: Epitaxial films of LiTi{sub 2}O{sub 4} on single crystalline substrates of MgAl{sub 2}O{sub 4}, MgO, and SrTiO{sub 3} provide model systems to systematically explore the effects of lattice strain and microstructural disorder on the superconducting state. Lattice strain that affects bandwidth gives rise to variations in the superconducting and normal state properties. Microstructural disorder, such as antiphase boundaries that give rise to Ti network disorder, reduces the critical temperature, and Ti network disorder combined with Mg interdiffusion lead to a much more dramatic effect on the superconducting state. Surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness.
Date: March 10, 2009
Creator: Chopdekar, R.V.; Wong, F.; Takamura, Y.; Arenholz, E. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Bonding, Backbonding, and Spin-Polarized Molecular Orbitals:Basis for Magnetism and Semiconducting Transport in V[TCNE]x~;;2

Description: X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V L2,3 and C and N K edges reveal bonding/backbonding interactions in films of the 400 K magnetic semiconductor V[TCNE]x~;;2. In V spectra, dxy-like orbitals are modeled assuming V2+ in an octahedral ligand field, while dz2 and dx2-y2 orbitals involved in strong covalent bonding cannot be modeled by atomic calculations. C and N MCD, and differences in XAS from neutral TCNE molecules, reveal spin-polarized molecular orbitals in V[TCNE]x~;;2 associated with backbonding interactions that yield its novel properties.
Date: May 20, 2008
Creator: Kortright, Jeffrey B.; Lincoln, Derek M.; Edelstein, Ruth Shima & Epstein, Arthur J.
Partner: UNT Libraries Government Documents Department

X-ray absorption spectroscopy at the Ni-K edge in Stackhousia tryonii Bailey hyperaccumulator

Description: Young plants of Stackhousia tryonii Bailey were exposed to 34 mM Ni kg-1 in the form of NiSO4- 6H2O solution and grown under controlled glasshouse conditions for a period of 20 days. Fresh leaf, stem and root samples were analysed in vivo by micro x-ray absorption spectroscopy (XAS) at the Ni-K edge.Both x-ray absorption near edge structure and extended x-ray absorption fine structure spectra were analysed, and theresulting spectra were compared with spectra obtained from nine biologically important Ni-containing model compounds. The results revealed that themajority of leaf, stem and root Ni in the hyperaccumulator was chelated by citrate.Our results also suggest that in leavesNi is complexed by phosphate and histidine, and in stems and roots, phytate and histidine. The XAS results provide an important physiological insightinto transport, detoxification and storage of Ni in S. tryonii plants.
Date: October 8, 2007
Creator: Ionescu, Mihail; Bhatia, Naveen P.; Cohen , David D.; Siegele, R.; Marcus, Matthew A.; Fakra, Sirine C. et al.
Partner: UNT Libraries Government Documents Department

Interface and magnetic characterization of ultrathin EuO films with XMCD

Description: We present work done on EuO films with thicknesses varying from 10 to 60 A grown as a stepped wedge on Si/Cr(20 {angstrom})/Cu(90 {angstrom}) and capped with Y(20 {angstrom})/Al(80 {angstrom}). The films were characterized by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the europium M{sub 5} and copper L{sub 3} edges. The films high quality and consistent magnetic properties were confirmed by SQUID magnetometry, which revealed a constant saturation moment independent of film thickness. XAS at the Cu L{sub 3} edge showed that the bottom Cu electrode is metallic (oxidation free). We report an XMCD intensity of 52% ({+-}4.3), in close agreement with theoretical calculations.
Date: September 20, 2008
Creator: Negusse, E.; Dvorak, J.; Holroyd, J. S.; Liberati, M.; Santos, T. S.; Moodera, J. S. et al.
Partner: UNT Libraries Government Documents Department

The structure of a-C: What NEXAFS and EXAFS see

Description: Mechanically hard ha-C and soft sa-C amorphous carbon films of 2.9 and 2.2 g cm-3 approximate densities were prepared by filtered cathodic arc deposition and analyzed by near-edge x-ray absorption spectroscopy NEXAFS and extended x-ray absorption spectroscopy EXAFS to determine their structure. The analysis observed an insignificant level of pi bond conjugation in both kind of films. EXAFS distinguished two types of atomic environments in them: one semiordered with well defined bond lengths, and the other with so strong bond disorder that its contribution to EXAFS was undetectable. The proportion of atoms in the semiordered atomic environments was of less than 40percent in both films. Their bond lengths were similar to those of diamond in the ha-C films and to graphite in the sa-C. NEXAFS spectra analysis was based on the linear relation between sigma* energy and bond length. It served to quantify the proportion of sp3 bonded atoms in a-C, to deduce the average bond length of the atoms undetected by EXAFS, and to determine the level of bond conjugation in the films. The sp3 concentration estimated with the proposed method was of 44percent in the ha-C films and 10percent in the sa-C films. These values were consistent with the EXAFS results, but disagreed with those obtained based on the traditional pi*/sigma* intensity ratio method which overestimated sp3 concentrations. Annealing of the ha-C films up to its almost complete graphitization caused a gradual reduction in bond length of the semiordered environments with no differentiation between two phases, diamondlike and graphitelike, at any temperature. This resultsupport models that explain sp3 bond promotion in a-C as caused by the high compressive stress attained by a strongly disordered sp2 dense structure during film deposition.
Date: August 1, 2006
Creator: Hussain, Zahid; Diaz, J.; Monteiro, O.R. & Hussain, Z.
Partner: UNT Libraries Government Documents Department

Probing Reaction Dynamics of Transition-Metal Complexes in Solution via Time-Resolved X-ray Spectroscopy

Description: We report measurements of the photo-induced Fe(II) spin crossover reaction dynamics in solution via time-resolved x-ray absorption spectroscopy. EXAFS measurements reveal that the iron?nitrogen bond lengthens by 0.21+-0.03 Angstrom in the high-spin transient excited state relative to the ground state. XANES measurements at the Fe L-edge show directly the influence of the structural change on the ligand-field splitting of the Fe(II) 3d orbitals associated with the spin transition.
Date: May 24, 2009
Creator: Huse, Nils; Khalil, Munira; Kim, Tae Kyu; Smeigh, Amanda L.; Jamula, Lindsey; McCusker, James K. et al.
Partner: UNT Libraries Government Documents Department

Multiplexed gas spectroscopy using tunable VCSELs

Description: Detection and identification of gas species using tunable laser diode laser absorption spectroscopy has been performed using vertical cavity surface emitting lasers (VCSEL). Two detection methods are compared: direct absorbance and wavelength modulation spectroscopy (WMS). In the first, the output of a DC-based laser is directly monitored to detect for any quench at the targeted specie wavelength. In the latter, the emission wavelength of the laser is modulated by applying a sinusoidal component on the drive current of frequency {omega}, and measuring the harmonics component (2{omega}) of the photo-detected current. This method shows a better sensitivity measured as signal to noise ratio, and is less susceptible to interference effects such as scattering or fouling. Gas detection was initially performed at room temperature and atmospheric conditions using VCSELs of emission wavelength 763 nm for oxygen and 1392 nm for water, scanning over a range of approximately 10 nm, sufficient to cover 5-10 gas specific absorption lines that enable identification and quantization of gas composition. The amplitude and frequency modulation parameters were optimized for each detected gas species, by performing two dimensional sweeps for both tuning current and either amplitude or frequency, respectively. We found that the highest detected signal is observed for a wavelength modulation amplitude equal to the width of the gas absorbance lines, in good agreement with theoretical calculations, and for modulation frequencies below the time response of the lasers (<50KHz). In conclusion, we will discuss limit of detection studies and further implementation and packaging of VCSELs in diode arrays for continuous and simultaneous monitoring of multiple species in gaseous mixtures.
Date: April 10, 2012
Creator: Bond, T; Bond, S; McCarrick, J; Zumstein, J; Chang, A; Moran, B et al.
Partner: UNT Libraries Government Documents Department

Contract B590089: Technical Evaluation of the Pu Cluster Calculations

Description: Using Synchrotron-Radiation-based Photoelectron Spectroscopy and X-ray Absorption Spectroscopy, the theoretical results within recent progress reports supplied under Contract B590089 have been evaluated. Three appendices are included: A is from Progress Report I; B is from Progress Report II; and C is from an earlier calculation by M. Ryzhkov. The comparisons between the LLNL experimental data and the Russian calculations are quite favorable. The Cluster calculations may represent a new and useful avenue to address unresolved questions within the field of actinide electron structure, particularly that of Pu.
Date: November 18, 2011
Creator: Tobin, J G; Ryzhkov, M & Mirmelstein, A
Partner: UNT Libraries Government Documents Department

X-Ray Absorption Spectroscopy of Uranium Dioxide

Description: After the CMMD Seminar by Sung Woo Yu on the subject of the x-ray spectroscopy of UO2, there arose some questions concerning the XAS of UO2. These questions can be distilled down to these three issues: (1) The validity of the data; (2) The monchromator energy calibration; and (3) The validity of XAS component of the figure shown. The following will be shown: (1) The data is valid; (2) It is possible to calibrate the monchromator; and (3) The XAS component of the above picture is correct. The remainder of this document is in three sections, corresponding to these three issues.
Date: December 10, 2010
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

In situ soft X-ray absorption spectroscopy investigation of electrochemical corrosion of copper in aqueous NaHCO3 solution

Description: A novel electrochemical setup has been developed for soft x-ray absorption studies of the electronic structure of electrode materials during electrochemical cycling. In this communication we illustrate the operation of the cell with a study of the corrosion behavior of copper in aqueous NaHCO3 solution via the electrochemically induced changes of its electronic structure. This development opens the way for in situ investigations of electrochemical processes, photovoltaics, batteries, fuel cells, water splitting, corrosion, electrodeposition, and a variety of important biological processes.
Date: March 31, 2010
Creator: Jiang, Peng; Chen, Jeng-Lung; Borondics, Ferenc; Glans, Per-Anders; West, Mark W.; Chang, Ching-Lin et al.
Partner: UNT Libraries Government Documents Department

Comparative Study of f-Element Electronic Structure across a Series of Multimetallic Actinide, Lanthanide-Actinide and Lanthanum-Actinide Complexes Possessing Redox-Active Bridging Ligands

Description: A comparative examination of the electronic interactions across a series of trimetallic actinide and mixed lanthanide-actinide and lanthanum-actinide complexes is presented. Using reduced, radical terpyridyl ligands as conduits in a bridging framework to promote intramolecular metal-metal communication, studies containing structural, electrochemical, and X-ray absorption spectroscopy are presented for (C{sub 5}Me{sub 5}){sub 2}An[-N=C(Bn)(tpy-M{l_brace}C{sub 5}Me4R{r_brace}{sub 2})]{sub 2} (where An = Th{sup IV}, U{sup IV}; Bn = CH{sub 2}C{sub 6}H{sub 5}; M = La{sup III}, Sm{sup III}, Yb{sup III}, U{sup III}; R = H, Me, Et) to reveal effects dependent on the identities of the metal ions and R-groups. The electrochemical results show differences in redox energetics at the peripheral 'M' site between complexes and significant wave splitting of the metal- and ligand-based processes indicating substantial electronic interactions between multiple redox sites across the actinide-containing bridge. Most striking is the appearance of strong electronic coupling for the trimetallic Yb{sup III}-U{sup IV}-Yb{sup III}, Sm{sup III}-U{sup IV}-Sm{sup III}, and La{sup III}-U{sup IV}-La{sup III} complexes, [8]{sup -}, [9b]{sup -} and [10b]{sup -}, respectively, whose calculated comproportionation constant K{sub c} is slightly larger than that reported for the benchmark Creutz-Taube ion. X-ray absorption studies for monometallic metallocene complexes of U{sup III}, U{sup IV}, and U{sup V} reveal small but detectable energy differences in the 'white-line' feature of the uranium L{sub III}-edges consistent with these variations in nominal oxidation state. The sum of this data provides evidence of 5f/6d-orbital participation in bonding and electronic delocalization in these multimetallic f-element complexes. An improved, high-yielding synthesis of 4{prime}-cyano-2,2{prime}:6{prime},2{double_prime}-terpyridine is also reported.
Date: February 24, 2010
Creator: Schelter, Eric J.; Wu, Ruilian; Veauthier, Jacqueline M.; Bauer, Eric D.; Booth, Corwin H.; Thomson, Robert K. et al.
Partner: UNT Libraries Government Documents Department

Atomic resolution mapping of the excited-state electronic structure of Cu2O with time-resolved x-ray absorption spectroscopy

Description: We have used time-resolved soft x-ray spectroscopy to investigate the electronic structure of optically excited cuprous oxide at the O K-edge and the Cu L3-edge. The 400 nm optical excitation shifts the Cu and O absorptions to lower energy, but does not change the integrated x-ray absorption significantly for either edge. The constant integrated x-ray absorption cross-section indicates that the conduction-band and valence-band edges have very similar Cu 3d and O 2p orbital contributions. The 2.1 eV optical band gap of Cu2O significantly exceeds the one eV shift in the Cu L3- and O K-edges absorption edges induced by optical excitation, demonstrating the importance of core-hole excitonic effects and valence electron screening in the x-ray absorption process.
Date: May 2, 2010
Creator: Hillyard, P. W.; Kuchibhatla, S. V. N. T.; Glover, T. E.; Hertlein, M. P.; Huse, Nils; Nachimuthu, P. et al.
Partner: UNT Libraries Government Documents Department

Room Temperature Magnetic Barrier Layers in Magnetic Tunnel Junctions

Description: We investigate the spin transport and interfacial magnetism of magnetic tunnel junctions with highly spin polarized LSMO and Fe3O4 electrodes and a ferrimagnetic NiFe2O4 (NFO) barrier layer. The spin dependent transport can be understood in terms of magnon-assisted spin dependent tunneling where the magnons are excited in the barrier layer itself. The NFO/Fe3O4 interface displays strong magnetic coupling, while the LSMO/NFO interface exhibits clear decoupling as determined by a combination of X-ray absorption spectroscopy and X-ray magnetic circular dichroism. This decoupling allows for distinct parallel and antiparallel electrode states in this all-magnetic trilayer. The spin transport of these devices, dominated by the NFO barrier layer magnetism, leads to a symmetric bias dependence of the junction magnetoresistance at all temperatures.
Date: March 9, 2010
Creator: Nelson-Cheeseman, B. B.; Wong, F. J.; Chopdekar, R. V.; Arenholz, E. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Investigations of Magnetic Overlayers at the Advanced Photon Source

Description: Magnetic overlayers of Fe and Co have been investigated with X-ray Magnetic Circular Dichroism in X-ray Absorption Spectroscopy (XMCD-ABS) and Photoelectron Spectroscopy (PES), including Spin-Resolved Photoelectron Spectroscopy (SRPES), at Beamline 4 at the Advanced Photon Source (APS). Particular emphasis was placed upon the interrogation of the 2p levels of the Fe.
Date: June 26, 2009
Creator: Tobin, J G; Yu, S & Butterfield, M T
Partner: UNT Libraries Government Documents Department

Measured chromium distributions resulting from cooling tower drift

Description: Useful data concerning the distribution of drift from a mechanical draft cooling tower were derived from field measurements despite the non-ideal conditions which were encountered. Observed chromium fluxes ranged from about 1 mg (m/sup 2/hr)/sup -1/ at a distance of 30 m from the tower to about 1% of that value at a kilometer. Air concentrations of chromium were fairly constant at about 50 ng m/sup -3/ to a distance of about 200 m downwind of the tower, apparently due to thorough mixing in the wake of the tower and a lack of lateral dilution of the effiuent from a crosswind line source. A simplified droplet trajectory model appears capable of estimating drift deposition flux within an order of magnitude. Neither that model nor the experimental data are sufficiently detailed to permit definitive assessment of cooling tower drift. The proper assessment of cooling tower drift deposition for general application would require a comprehensive investigation centered on an isolated cooling lower whose operational characteristics are well-defined and subject ulo cont rol. Documentation of the drift droplet spectrum and properties of the buoyant plume and the adjacent atmosphere are essential to such an effont. (auth)
Date: January 1, 1973
Creator: Alkezweeny, A.J.; Glover, D.W.; Lee, R.N.; Sloot, J.W. & Wolf, M.A.
Partner: UNT Libraries Government Documents Department

Low-Dimensional Water on Ru(0001); Model System for X-ray Absorption Spectroscopy Studies of Liquid Water

Description: We present an x-ray absorption spectroscopy results for fully broken to a complete H-bond network of water molecules on Ru(0001) by varying the morphology from isolated water molecules via two-dimensional clusters to a fully covered monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H-bonding is further elucidated for the amino (-NH{sub 2}) group in glycine adsorbed on Cu(110) where the E-vector is parallel either to the NH donating an H-bond or to the non-H-bonded NH. The results give further evidence for the interpretation of the various spectral features of liquid water and for the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems.
Date: May 11, 2009
Creator: Nordlund, D.; Ogasawara, H.; Andersson, K.J.; Tatarkhanov, M.; Salmeron, M.; Pettersson, L. G. M. et al.
Partner: UNT Libraries Government Documents Department